I didn't read all the replies but from what i understand for spell hit/crit is that the roll is hit/resist(partial)/crit. So at certain point hit = crit. A fully resisted spell could have been a crit had it not been resisted.

I know i might get some arguments here because i know the melee roll system is different.

I know on my old raiding warlock with 30% crit (character sheet+desto talents) my 30% crit wasn't 30% on bosses until i stacked enough hit.