Run some sample dives on Subsurface with your SAC and GF99/75. Find out predicted gas consumption with varying depths and SS lengths.
Come up with a gas requirement based on Time to Surface. For me, it's about 1.2 CuFt/min TTS, decreasing a bit with deeper dives. Add your safety factor, and maybe double it if you want enough gas for your OOA buddy, and you have a rock bottom model that is based on GF.
Takes a little playing on the computer, but it solves your issue completely. Track your evolving TTS based on 99/99. Add the extra SS minutes that are required to get you to 75. That's your revised TTS.
Revised TTS x 1.2 = my required gas reserve in CF not including safety margins.