well 9:30 is more of a push then 8:15 logically, I'd probably go with the longer time for a 3 stop push given how muhc of tmax400s shadows just suddenly drop out on me when it was underexposed developing normally in xtol (was someone elses roll and they didnt tell me).. otherwise i would have pused it
Why not try the manufacturer's suggestion first? These guys made that film and developer and they have done far more testing than anyone else out there. Obviously, their suggestion isn't some kind of law and they do state it at their technical publications for any film, so you should judge the results and fine-tune afterwards. From some point on it becomes a matter of taste.
Do not be afraid. Follow 9:30 at 24°C. I did that with the right result. I did that 9 1/2 min after a pre-soak 30" at same T°C and the start timing when a first agitation cycle is finished.
Sounds great, thanks guys for the comments. Will the film fog up since its being developed for so long? Anything I can do to decrease the fog? What should I do with agitiation, 5 agitations per 30 seconds makes me doubtful