I was just watching a speerun of FFT by eLmaGus on RPGLM2017, and I thought I'd make a comment based on a comment from the couch--in case it might be helpful.

If an effect has a success percentage of succeeding of 74%, then casting it twice doesn't give you a 148% chance of getting the effect--thus guaranteeing that you would get it. The chance is 93%--i.e. there is still a chance, albeit small, of not getting the effect after 2 casts. I believe this was noted in the context of the Quick ability.

0.74 + (1-0.74)*0.74

This probability is recursive for subsequent casts.

I hope this helps,

Ron Gorzon