On the other hand, thanks to help from @jqr and a healthy does of trial and error, I've gotten that pesky statistic bonus problem figured out. This is what ended up working:
fortitude_base + character.statistic.con_modifier + magic + misc
(constitution.to_i - 10) / 2
I'm keeping it as .to_i now, as a fix for a case where a player tries to create a Fortitude save before a Constitution score has been entered. The .to_i translates a nil record to 0, so Rails does a temporary calculation (which isn't shown) rather than displaying an error. Not sure if there's a better way to handle this (perhaps simply checking that statistic.constitution.present? before performing any calculations) but it works for now.
More importantly than simply figuring this one instance of how to define logic in one model and then call it in the view of another is understanding HOW it works. I can extrapolate these methods and apply them to other things: skills, attack rolls, armor class, etc, etc. Once again, Manticore proves to be a useful learning tool! SUCCESS!