http://forum.square-enix.com/ffxiv/t...age-Calculator

But I'll sum it up for you

Basically: If you do 640 damage on average, crit > determination. Spell speed is never bad as a black mage, but crit outweighs spell speed to a certain extend. In a nutshell: Crit >>> Spell speed > Det if you do more than 640 damage on average. (which is easily obtained with i85 gear)