I just did some spitball stuff with 354 crit vs 888, and the damage formula seems to be relatively close, and redo'ing with math it appears a sliding stat weight for crit.
Using their figure for crit hit chance (high/low damage is easy to get, not crit chance), what I come up with is with +0 Crit, you'll need about +91 to get a 1% boost to damage. At around +800 crit, you'll get that 1% boost from about +60 crit.
These figures would hold true no matter what your INT or WD are, same as Spell Speed.
