I think I figured it out. Fas DOES add 20 crit chance, however monsters have crit def as well based on their luk. Every 5 luk gives 1 crit def. So acidus for example has 69 luk with gives it roughly 14 crit def, meaning I need 114 crit (94 + 20 from Fas) to crit acidus 100% of the time.
Can somebody confirm if this is true?