Now that I have made about 40% of BG1 monsters, I am not satisfied with experience value.
No consistency at all and some strange numbers.

P&P doesn't help here. For example, according to P&P, hamadryad worth less than dryad but if you look closer, hamadryad is better in all points (HD, spells, ...)

So, I will add an optional new experience system for monster.

How it will work?
Something like: HD * HDValue * PowerCoeficient
HD = hit dice
HDValue = value of one hit dice, to be determined
PowerCoeficient = based on monster's threat

Basically, each monster will have a threat number between 1 and 5, 1 = very easy, 2 = easy, 3 = normal, 4 = hard, 5 = very hard
Then this number is converted into a coeficient.

For example, let's take some monsters:
- Kobold, 1HD (half HP), difficulty number = 1
- Hobgoblin, 1HD, difficulty number = 3
- Dryad, 2HD, difficulty number = 4 (magic resistance and 3 powerful charm spells)
- Battle Horror, 9HD, difficulty number = 5 (powerful weapon (2D6), many immunities, great strength)
[...]

It won't be perfect but I think it will be better than current broken system.