From what I recall from when I dabbled in the script files, damage calculation is more or less as follow:
Strength is your %chance of doing a critical hit (doubled damage iirc).
Ballistic Armor is the %chance of reducing the attacker base+crit damage (halving it).
Penetration is another %chance of extra damage (as crit hit) that ignores ballistic armor.
Heavy armor is another %chance of reducing the attacker total damage (damage and penetration damage).
I'll check back when I'll be in front of my gaming computer, but it works as follow:

Heavy armor reduces damage to 1/2 if the random roll is <= HA
It halves damage another time if random roll <= HA-20
and a third time if random roll <= HA -40.
All these reductions are multiplicative (so damage is quartered if <HA-20, and divided by eight if <= HA-40)
Also, if Penetration is under 40 and Heavy Armor is >=40 then another Heavy armor roll is made, (even if the first one was succesful), so the max damage reduction from heavy armor is to divide damage by 64!

Heavy armor is by far the most important element, as getting extra points significantly increase the chance of reducing damage, and the amount of damage reduced (while higher penetration is just straight extra damage and doesn't impact the chance to reduce damage).
Also, if HA is >=100, then you are guaranteed to take half damage at most (and you have more than 60%chance if taking 1/8 damage).
That is also the reason why rear armor dies to everything. a 80+ front armor is almost guaranteed to stop most damage, but 40- rear armor cannot get the quartered damage result.

Note that special ammunition and melta don't follow the exact same formula. I don't remember the exact formula, but from what I recall, they reduce the heavy armor.

This formula is somewhat similar to the way things work in epic Armageddon(in EA, most units have a single save roll. Reinforced units can save twice. Macro weapons ignore the first save, and Titan killer weapons like the volcano also ignore the second save). It makes armor rather consistent, but it also makes it impossible to have "consistent" weapons (aka, even with 100 Penetration, you still have 60% chance of doing 1/8 damage), unless they fall under the special rules.
I think it is OK for most weapons (maybe even the lascannon, because it didn't have any special rule in Epic Armageddon either), but I think super heavy weapons (volcano cannon, thunder hammer, supakannon) should work like melta and reduce heavy armor as it is the only way to make damage more reliable.