mega tight fit, for like lambskin rider's jackets and stuff (this is like, Julius-level tight) - your chest/2 * 1.03-1.05 - (1.03 means you better be skinny and not breathe much)
slim fit = your chest/2 * 1.08
'normal fit' = chest/2 * 1.10
'relaxed fit = chest/2 * 1.12ish
So for me 40.75" chest divided by 2 times 1.08 = 22.005 pit to pit measurement.