The math behind skill increases has been thoroughly worked out. If I remember right, it uses the base value but the condition for a rank up is different on uniques (must reach the next integer) vs legendaries (must simply reach 40% to next value).
I don’t know why it’s different for both. But I also just kinda hate it. I crit twice on a skill on a unique amulet and the skill didn’t rise. It was totally wasted and now I have to blow mats and 5 million gold to reset it because I got nothing for my “luck”.
Really feels like critting on a skill should always raise it by 1, and otherwise it should go up every 3 hits (9 hits total because 3 are taken up by crits). So by the end, a skill goes up anywhere from 3 (no crits) to 6 (all crits). This would be regardless of the base value.
Is it OP? Yeah, it definitely is. So up the base masterwork increase from 5% to something slightly higher to compensate. But it also makes your time/“luck” spent critting the damn thing feel worth it. As it is an UTTER WASTE OF TIME AND MONEY critting twice on a skill on a legendary and I don’t understand how that got past QA because in my opinion it makes ZERO sense from a player perspective. It feels like a mistake.