The big problem for SBF I think is that he's been shown to have "no filter", or no regard for accepted norms. That could bite him even in federal prison (and he has to go to a medium security, I believe, because of the length of the sentence).

Piss off an inmate and get in an even slight altercation, lose some of your good behavior points.

12.5 years would be possible only if he scored 100% on good behavior.