I got you, but don't have as such formula on top of my head.
I never even think about any formula in calculating my next extent size from the fragmentation point of view. I will make sure that I will end up having my Initial and Next extents sizes are same always and thats how I try to avoid fragmentation.

Same case with percentage increase and I will keep it at Zero.
This is what big guys/Oracle Support advise... you know !

I don't understand one thing here. Are you trying to caluculate this stuff for documentation and reporting purpose or creating the Objects with redefinded storage clauses?

Sorry ! Im not of much help to you...