To go from level 49 to 50 takes (1.2)^25 = about 95.4 times as much experience as going from level 24 to 25.

To actually calculate the total amount of XP needed, we would need to sum the geometric series.

If we ignore the fact that the pattern is different for the first 10 or so levels, you are only about .5% of the way there, in terms of XP.

After a certain point, the only place you will find enemies that give you enough XP to level up in anything resembling a reasonable amount of time is Ascension Peak.

I believe that the Mt. Gigas caves have some tougher level 20 something spiders, tougher bats and Moraxes, etc. The Rapax areas have tougher enemies too. Not counting Ascension Peak, the Mountain Wilderness probably has the best experience, with hard battles with good experience (Acid Wings, fairy enemies, Molten Scorchers, etc.) I recall getting an Estoc de Olivia, or whatever it's called, for my Ranger, which is probably the best one-handed sword they can equip (very rare) from that Rapax harassment group wandering around there.
Ascension Peak has some very annoying and difficult encounters (rightfully so, as the last area in the game). Those gazers can cause status problems for the party (Turncoat being particularly annoying). In my previous party, sometimes level 50 Armegazers (sp) would spawn, thankfully one at a time. The only good way to kill them was with the Ranger's instakill.
I would probably be happy with level 35-40 before going in for the final plunge on Ascension Peak.

Level 26:
Priest: (Spellbook complete)
Alchemist: Blinding Flash
Psionic: Sleep
Mage: Turncoat
Levelling up is starting to get difficult, due to exponential experience requirements. It took almost 5 million experience points to go from level 25 to 26 for the specialist casters. At least now, I will get better potential treasures when I reach new areas from the treasure chests that actually have a level sensor. I'm in the 2nd best group now. The best group is level 31+. The reason I bring this up is because I recall Trynton having at least two good chests, one being in Trynton ground level, in that room hollowed out of a big tree (just before the Marten's Pagoda bridge that collapsed, and the gondola that brings you up to Trynton Upper Branches). The other being in the Fourth Bough, I believe, in that big room where you have to light the 4 incense braziers. Near there is a tough battle with some kind of tree enemy; Behind them is a room with good treasure chests. Also, I believe the building where Don Barlone and Milano Calzone are has good chests too.
I will probably leave Arnika soon. Assuming a group of Savant Slashers (a common enemy at my level) gives 100K experience to each team member, it would take 50 battles to get 5 million experience points. That takes a long time, needless to say, and that's considering that the good, high-experience battles only seem to occur during daytime hours (I get low experience point Higardi bandits at night).
I have probably only completed around 5-10% of the game, having cleared the Lower and Upper Monastery, Arnika Road, and Arnika of all treasures and set battles (not counting gaining access to the Mook headquarters). It's probably about time I get moving on, considering that I'm near endgame level. Most of the rest of the game should be easy at this point, and my low HP casters won't be so vulnerable at a higher level.
My next destination, being an MDP, should obviously be Trynton, for the fountain riddle that gives a permanent +5 to Intelligence, unlocking Powercast for my casters (they have been patiently waiting at 95 Intelligence for an eternity). Powercast is easy to train, thankfully, since casting any spell will do it. Playing catch-up shouldn't be too hard.
I will admit, I am playing on Novice difficulty. I've ground levels and skill trained so heavily that the lack of Powercast isn't too bad. I can wipe out all enemies in just one round, with the exception of tougher Savant machines such as Savant Berserkers (level 17), Savant Destroyers (level 18), Savant Slashers (level 21), and Savant Minion (level 22). They have high resistances, and the lack of Powercast resistance piercing forces me to fight physically, both melee and ranged.
This is not necessarily a bad thing, as fighting the Savant Berserkers and Slashers has made my Bard, Gadgeteer, Priest, and Alchemist decent at close combat (my Gadgeteer got her Shield skill to 100 last night, aided by the Heater Shield's +5 to Shield skill, she's now at 105 Shield skill). Whether Shield skill above 75 for the +3 AC Heater Shield does anything is beyond me, but common knowledge says that the higher the Shield skill, the better the chance to block. Indeed, I have noticed a lot of blocking from my Gadgeteer. Thank goodness monsters don't use shields.
Also, fighting the Savant Destroyers and Minion at range has made all 6 characters good at ranged combat. The Bard and Gadgeteer fire off a lot of Quarrels with their Tripleshot Crossbows, and their Ranged Combat and Bow skills are near maxed. The casters seem to only be able to hit reliably around thrown range with their slings, due to having neglected Strength and Dexterity, and lack of Eagle Eye (at least their Ranged Combat and Throwing and Sling skills are relatively high, and the KO% is always welcome when it lands).
According to Flamestryke's experience chart, staying in Arnika until level 31 would be almost impossible, due to the exponential growth of experience points needed to level, so time to move on.

I lied. The level up split down the middle my skill grinding. My Bard managed to get to 100 Shield skill, as well as the Gadgeteer. Both the Bard and Gadgeteer are at 99 Sword skill. The Gadgeteer is at 99 Bow skill, and the Bard at 98 Bow skill. I believe that getting to 100 skill will give them an extra attack, or swing (it should, as getting from 99 to 100 skill is the hardest thing in the game, besides farming things like Light Swords and Light Shields).
Hanging out in Arnika this long has made me notice strange things. Not only do Higardi Raiders/Roustabouts/Cutpurses/Highwaymen/Brigand spawn in regularly at night, but sometimes, extremely rarely, help will arrive. A team of 3 or 4 Higardi Patrol will spawn in for a while, until they disappear, much like how enemies despawn. This is much like the other areas, where various Umpani, T'Rang, Rapax, Rattkin, Trynnie will spawn in their home areas.
I noticed a one in a million chance, or so it seems. I was waiting by the crash site for enemies to spawn (it was after midnight), and a bunch of rats spawned (a mix of 11 Rabid Rats and Gnawer Rats). I have never seen that happen in the 22 years this game has been out for. I just thought I would pass that along. This game is full of surprises that even us veterans notice.

Level 27:
Priest: (Spellbook complete)
Alchemist: Purify Air
Psionic: Prismic Ray
Mage: Blizzard
A very exciting level up for my Psionic! He maxed Speed a long time ago, and his Snakespeed skill is above 80, granting +5 to initiative. He just maxed Senses and Piety, unlocking Eagle Eye and Iron Will. Intelligence will be maxed at the Trynton Fountain, unlocking Powercast (easy to play catch up, and I've been fine without it thus far). I'm only becoming more powerful in relation to the enemies.
The Bard and Gadgeteer have maxed both their Sword and Shield skills, thanks to their controlling attributes (Strength and Dexterity) being at 100. Close Combat, Powerstrike, and Reflextion are all in the mid 90s.
Edit: I noticed that selling almost all of my items to get to white encumberance allows the Bard and Gadgeteer to get 3 swings per attack with their swords, instead of the 1 or 2 swings they get in blue encumberance. Also, they can get 3 arrows off at range (modified by +2 shots from the Tripleshot Crossbow), for a total of 5 shots per attack. It's hard to stay in white though, I wouldn't be able to carry around more than a few items before hitting blue.

Powercast close to 100 compared to close to 1
raises the max dmg output of spells by 25%. above the theoretical max.
It also helps to pierce target resistance and helps vs high lvl targets.

So i dont get why to delay the well,
at lvl 20 there were already no monsters spawning on the way that may pose a potential threat.

"encumbrance"
Reduce ammo in personal inventory, e.g. casters dont need to shoot at all
(is also a violation of KWM, and elementals are better anyway)
Also could consider to trade in 1 or 2 AC for some lbs of saved weight, or just go for Ferro.

btw, did you get Ariels slippers?

Powercast also, for the record:
* Increases the power of healing spells. (Given how few healing spells there are in the game, after a point this becomes the only way to make healing, particularly multi-target healing, stronger.) I believe this also increases the power of Stamina/Rest All.
* Increases the power of many other combat spells, including things like Haste/Superman. I believe Guardian Angel is also included.
* Increases the duration (but not the power) of spells that work outside of combat, including Light with its (normally) fixed duration.

Edit: Also worth noting that, with expert skills like Power Cast:
* Having 0 in the skill is better than not having it at all.
* If you don't meet the requirements, but somehow have a non-zero value in the skill, it won't function. (This can happen either due to equipment, like how those Snakeskin Boots boost Snake Speed, which only has an effect if you already have base 100 speed, or if your character gets diseased and suffers stat loss as a result.) (Note that this is not how non-expert skills work.)

I'm going for a no-death run. My 4 casters are ignoring fighter-oriented stats such as Strength and Dexterity (and Vitality, which isn't a bad stat, but it helps no skills rise) in favor of going first and being powerful casters in general (Speed, Senses, and Piety being maxed first). Intelligence was left at 95 for all 4, because I'm a baby, and I refuse to waste the 5 points until I hit the Trynton Fountain.
I'm probably the heaviest grinder on the GOG forums. I can't stand losing characters, so I made them super tough (level 21) before the first Arnika Road run. Once getting to Arnika, I got addicted to the easy experience of Savant Slashers and Minions (around 100K experience per character per battle, more or less). Note that Arnika spawns no magic-using enemies, and there are plenty of places to hide, and rest.
I'm trying to make characters that are good at everything: Magic, Ranged, and Melee (except for the physically weak Psionic and Mage; They can sling stones for KO% and or cast magic).
Note that without Bloodlust, the Bard and Gadgeteer are only mediocre warriors, nothing like the Fighter and Rogue, with their Berserking and Backstabbing. They, along with the Priest, are only there to "hold the line" with their weapon and shield, protecting the Alchemist, Psionic, and Mage in the center with no shields. This is done with the Bard in the front formation, and the Gadgeteer and Priest on the left and right flanks, with my back to a wall, if possible. The Alchemist's one-handed weapon options are pathetic, just like the Psionic and Mage, so might as well have my Alchemist ditch the shield, and use a big two-handed, extended range Quarterstaff to train for the Staff of Doom later.
You are correct that I could ditch the heavy Spike Stones, but they do give a KO% chance. Also, Powercast isn't unlocked yet, so against magic resistant enemies like Savant Slashers, some physical damage is required. A Bishop, arguably, is the only primary casting class that would pull her weight with no weapon skills, due to being so versatile. However, they would start to really lag behind in levels with this MDP. Right now, the Bard and Gadgeteer are never more than a level ahead of the specialist casters.
I found the Ariel's Slippers a long time ago, along with the Prospero's Cloak and Mercucio's Blade from the thieves in the Monastery.
I was hoping that Powercast would double the magic damage of my casters, but that would perhaps be too powerful. I would be happy with a 50% increase in effectiveness. I realize that PC helps healing spells too, which is why even my Priest will get it.
I heard that expert skills suffer a 50% penalty to chance to increase skill., making them harder to max. This would be similar to having their controlling attribute at 50.

You're not wrong. I might just set out for Trynton tonight. At least I can hit the fountain, max Intelligence, and unlock Powercast, then set a portal there, and just portal back to Arnika to grind out some Powercast.
Edit: I just did a major run where I hit the Trynton fountain.
Arnika-Trynton Road: Besides the random loot lying around, I grabbed the hidden two items near the T'Rang house. I looted the chest that is in the deep crevice. I whacked Rattus Rattus, after he set me up for a bank robbery (I will have to kill Milano and the Don, so be it). At least I won't have to pay for the Astral Dominae.
Trynton: I killed a bunch of Hogars and Large Spiders and Picuses. I also cleared the set spawns in the tree. The good treasure chest (the one at the top in the tree) gave me a Wrist Rocket.
Trynton Upper Branches: I answered the riddle, granting 100 to Intelligence for my casters, and Fuzzfas got my Bard to 78 Communication (97 after the bonus is applied). This was after talking to 3 NPCs (Rattus Rattus, Chief Gari, and Fuzzfas).
Time to work on Powercast now.

Level 28:
Priest: (Spellbook complete)
Alchemist: Chameleon
Psionic: Pandemonium (Spellbook complete)
Mage: Armormelt
This is the first level up since I unlocked Powercast for my 4 casters. Everybody has this skill to at least 20 (30 for the Priest, since he casts so many buffs, and Stamina and Rest All). The Priest and Mage have an advantage here with casting buffs. At least the Alchemist can cast Light and Chameleon, while all the Psionic gets is Chameleon. The Alchemist and Psionic will alternate recasting it, since its duration is relatively short. Speaking of Chameleon, it really seems to help with giving me more time to re-position my party before I'm spotted by enemies.
Powercast is also unique among the expert skills, in that its benefit seems to be linear, so grinding it as high as possible always helps. Most other expert skills (Powerstrike, Iron Will, Iron Skin, Reflextion, Snake Speed, Eagle Eye) seem to give benefits at certain thresholds. I have noticed the durations for buff spells got longer, since I unlocked Powercast.
A minor note: At level 28, you get the best title for your characters. This is on the character screen, where it says: Level 28 Bard (Muse), for example. I guess this is around the time when the developers thought you should be near the end of the game.
The Psionic is at the top of his career. He has the best title, has picked every spell in his spellbook, and has maxed Intelligence, Piety, Speed, and Senses, unlocking Powercast, Iron Will, Snakespeed, and Eagle Eye. This level up I just put two points each into Strength, Vitality, and Dexterity. Might as well get a little bit of everything.
The Bard, Priest, and Mage will max 4 attributes next level, at level 29. The Gadgeteer and Alchemist, with their heavier profession requirements, will have to wait until level 30 for that. At least, as I level up and get more powerful, I'm simultaneously training up Powercast.

Level 29:
Priest: (Spellbook complete)
Alchemist: Sleep
Psionic: (Spellbook complete)
Mage: Shrill Sound
The Bard maxed Speed and Senses, unlocking Snake Speed and Eagle Eye. The Priest and Mage maxed Piety and Senses, unlocking Iron Will and Eagle Eye.
Powercast is in the 40s for all but the Priest (50s), protecting the party with Guardian Angel, and Stamina and Rest All so others can do the fighting.
Close Combat, Powerstrike, Reflextion, and Bow skills are stubbornly stuck at 98 or 99 for the Bard and Gadgeteer. The RNG gods have not blessed me with 100 skill yet.
Despite the longer time it takes for each consecutive level up, those Savant Slashers and Minion are still worth fighting, for melee and ranged combat skill increases, respectively.

Level 30:
Priest: (Spellbook complete)
Alchemist: Razor Cloak
Psionic: (Spellbook complete)
Mage: Shadow Hound
Finally, all of the attributes have been maxed, and all of the expert skills have been unlocked! The Gadgeteer maxed Speed and Senses, unlocking Snakespeed and Eagle Eye. The Alchemist maxed Piety and Senses, unlocking Iron Will and Eagle Eye.
This is due to their "useless" attributes of the Gadgeteer's 55 Intelligence (60 with the Trynton well), as well as the Alchemist's 60 Dexterity. Neither Intelligence for the Gadgeteer, nor Dexterity for the Alchemist, seem to be optimal attributes.
The Bard and Gadgeteer have maxed Strength, Dexterity, Speed, and Senses, unlocking Powerstrike, Reflextion, Snakespeed, and Eagle Eye. This, I believe, maxes them optimal fighters, both in melee and ranged combat.
The 4 specialist casters have maxed Intelligence, Piety, Speed, and Senses, unlocking Powercast, Iron Will, Snakespeed, and Eagle Eye. This, I believe, makes them optimal casters, both in magic attack and defense, and ranged combat. Melee combat for the casters is a waste, except for the Priest and Alchemist with The Mauler/Diamond Eyes and Staff of Doom, respectively. Even then, they won't be as good as even a Bard or Gadgeteer.
Melee Combat: The Bard and Gadgeteer managed to get 100 skill in Sword, Shield, Powerstrike, and Reflextion skills. Their Close Combat skill is stuck at 99, because I ignored Intelligence, one of the controlling attributes. The Alchemist managed to get 100 skill to both Close Combat and Staff and Wand skills, simply by swinging from the back line, in the center formation, protected, along with the Psionic and Mage. I sold the Quarterstaff, as training is over, the Alchemist will never get Powerstrike, and swinging for 1-5 damage is a waste. He should be ready for the Staff of Doom. The Priest is stuck in the 80s and 90s for his melee skills, as his attributes do not reflect melee combat. Even still, he will be ready to swing The Mauler/Diamond Eyes with a decent chance of success.
Ranged Combat: The Bard and Gadgeteer are decent in Ranged Combat and Bow skills. The four casters are decent in Ranged Combat and Throwing and Sling skills. Everybody has unlocked the Eagle Eye expert skill. The Mage has a Wrist Rocket, as she is the best at ranged combat.
Magic Combat: The four specialist casters have Powercast skill at 60 or better. The Bard and Gadgeteer, this early in the game, sold most of their useless instruments and gadgets. The Bard still has her Bless instrument.

I must admit, I like reading those posts and they kinda remind me of good times with Wiz8. Maybe even someday I'll try something similar myself (excluding the endless grind for attributes).
As for the quote - and as far as I remember - DEX also used to be important for ranged combat, and you didn't raise it for your casters. Why?

By the way, thanks for the kick in the @$$ to leave Arnika for a bit and hit the Trynton well. I would maybe have wasted time in Arnika not grinding up Powercast. The perfectionist in me refuses to waste the 5 precious attribute points in Intelligence before the well.
This is maybe my favorite game of all time. I enjoy the grind, frustrating as it may be, after a long night at work. I'm actually doing my own version of MPNorman's MDP example, except that the Priest and Alchemist are focused on magic, not physical combat. MDPs are actually fun, except for the early game grind, when you're weak, and have little spell points, and the dreaded Arnika Road first run.
You can't max all the attributes. I can get as close as possible with an all-human party though. According to my calculations, at level 50, I could have 6 attributes maxed, and the 7th at 74 (pick a dump stat). That's just insanity though. Am I to hang around Ascension Peak, waiting around for an eternity for level 50 enemies to spawn, and hope for an instakill?
I considered carefully, before starting my MDP, which attribues to max first, and which to forget about. You have to specialize in this game. Trying to even out your attributes, at least early on, will lead to a weak party with no expert skills. At this point, I can start to even out the remaining 3 attributes for my party that are not maxed yet.
The Bard and Gadgeteer are the fighters in this particular party, so I chose to max Strength and Dexterity first (Powerstrike and Reflextion being obvious for front liners). Initiative isn't as important for them, so I maxed Speed and Senses after that (Snakespeed late will still let them go first, and Eagle Eye will be nice for their Crossbows to hit). That leaves Intelligence, Piety, and Vitality as least important. Intelligence is the most worthless, as they don't benefit from Powercast, and it doesn't do much anyways. Piety is second most worthless, as all it gives is stamina (though Iron Will would be nice, it's too costly). Vitality is the best remaining attribute to pump, as it gives hit points, stamina, and carry capacity (Iron Skin is ok, but you can get damage resistance from the Alchemist's Body of Stone spell).
The four specialist casters depend heavily on Intelligence and Piety for their skills' controlling attributes, and of course you want Intelligence for Powercast and Piety for stamina, spell points, and Iron Will. Since this is a MDP, you want to hit hard and fast (squishy characters, also magic protections first), so Speed and Senses get high priority (Snake Speed to go first, Eagle Eye so they can actually hit with their slings). This leaves Strength, Vitality, and Dexterity as dump stats. At this point, I might as well even out those three, because they will never max, unless I want to grind to level 50.