That's long! I didn't realize it was a shouldered bullet. It makes it longer, but also allows for a lot of length flexibility with your platform. It wouldn't work in a .357 magazine, though.
As far as the load recipe goes.....
I think I would start a little lower than 8.9 gr. While you are seating longer, the base will be similar to that of a FP or RN bullet of .357 compatible design, perhaps even deeper.
I would recommend no more than the stock RSA, or even lighter (15#). The G21 does well with the stock spring, but the slide is significantly lighter than the G20. If you have live-fire feed issues, try a G21 slide or a 15# spring.
Life is tough. It's tougher if you're stupid. -- The Duke