Skill up speed

[Trejir]

It feels like skill ups are few and far between despite a 95 dex and 100-something INT on my lizard. I combined 15 items tailoring, was successful on all but 4 and gained 1 skill point (from skill 2 to skill 3). My question is; is there a piece-wise function that determines 'if skill up?' and if so what is that?

Anecdotally, does this rate seem about par? Appreciate your time!