How do you evaluate Stat values

I'm wondering how you guys determine the value of a certain stat without actually just testing it?

Like, right now I'm thinking about +4 support skills and +1 all skills vs 30% ADDITIONAL origin effect (+some 30ish% regular effect). I honestly dont know what either of these stats do. +4 support gives a bunch of 1-2% increases to my skills, which is probably dope but i cant quantify it.

30% additional origin effect is a wild card - is it just +30% origin effect or does it multiply all my other effects by 30%? can we take the wording "additional" literally or is it just a terrible translation?

Why is 18% damage better than +30 int when im at 800 int and 500% +dam but the reverse is true when those values are different?

tldr: how are you making your builds?