Trying to make that skill calc, I ran into something:
When Clay Golem is lvl0, it shows Life: 375.
When Clay Golem is lvl1, it shows Life: 450.

Which is true?

Clay's base life is 300, 450 (NM), 600 (Hell)

Life per level = Base + 25% + lvl*25%
so 375 = Base + 25%
and 450 = Base + 25% + 1* 25%

Basically, in skills.txt it's a minor typo where the linear life gain per level should have been 0% + lvl*25% but instead reads 25% + lvl*25%. All golems share this formula.

All Golems base life is:
Clay = 300, 450, 600
Flesh = 400, 600, 800
Iron = 500, 750, 1000
Force = 600, 900, 1200

So a clay golem at level 1 will have 375 or 450 hp? Hp adjustment is 'ln56', which, as far as I know, means par5 * (lvl-1) * par6. Life is 375 this way, just as it is shown before putting a point into the skill. Confused here.

450 is the correct lvl1 life. For your calculator, this will be one of the easiest formulas you'll ever get, since it's:
HP = BaseHP * (1.25+(BaseLvl + SoftLvls)*0.25 + GMasteryLvl*0.1)

I don't have a handy reference for ln56, but it should be par5 + lvl*par6. Typically par5 is zero, but that isn't the case with Golems.

I see. Thank you.