Anybody got any info on how to calculate a character's hit points?

I figured it depends on Level, Class, and Vitality, but how exactly I don't know.

From the Cosmic Forge editor I got the following info regarding HP for the different classes:
Fighter: 9
Lord, Valkyrie: 7.5
Samurai: 7
Ranger, Ninja, Monk: 6.5
Rogue, Bard: 6
Gadgeteer: 5.5
Priest: 5
Alchemist: 4.5
Bishop: 4
Psionic: 3.5
Mage: 3

However, somewhere else I've found sometime a formula (I don't remember where though) for HP calculation saying:
HPmax = (1+lvl)/2 * (A+B*VIT/100), with A and B being constants depending on the respective class (e.g. Fighter: A=11, B=13.9; Ranger: A=8, B=10; Mage: A=4, B=4.9)

Now, I have no clue how these two info go together. Has anybody an idea?

I think it works something like this:

First, a character has what I'll call a base HP value. This amount increases by the Cosmic Forge editor value after level up and starts at twice the value. (Note that this means that your starting class counts double here.)

Then there is the vitality modifier. Your base HP is multiplied by (VIT + 75) and divided by 125. At 50 VIT, your HP will equal its base and it changes from there. A VIT of 1 (possible through untreated disease, I believe) will reduce your HP to just over 3/5ths, and a VIT of 125 (possible with equipment or Superman) will give you a 60% HP bonus.

Also, note that the game rounds rather than truncates here.

Yes, the divisor is 125, not 100. (I just tested it.) Hence, the formula you gave is actually incorrect, and the reason for the two constants is due to the formula not being the right one to begin with.

Hence, the Cosmic Forge figures are the correct ones.

Ok great, thank you.

So the formula would be something like this:

HP = C * (lvl+1) * (VIT+75)/125
(C is this class constant I listed in the opening post)

That's quite interesting, because class choice has quite a high impact on your hit points, in some cases way more than your Vitality.
For example, a Fighter and the comparable Lord (or Valkyrie): A Lord with VIT of 100 has about as much HP as a Fighter with a VIT of 70. That's 10 levels of attributes.