First of all, welcome to the 500 s&w club, I have the 8 3/8 barrel and love shooting THE hand cannon. Recoil is so mitigated by the compensator that it feels lighter than my 44 mag 629 with 5 inch barrel. But you know you're shooting something .50 cal and 50,000+ odd psi. What a gun!!!
How does the 4 inch barrel feel, I'm curious? Anyways, if you have a chrono, why don't you just do a ladder test from the starting load and see when you get to the velocity you desire, all the while checking for pressure signs etc. I shoot cast bullets and dont always fine the exact bullet weight, so see whats closest to that, which is what a hodgden rep told me a while ago, go with that. I've never had any problems, but i'm always super cautious developing a load. Just my opinion. Good luck.