I would start with either 85gr of pyrodex, or 80gr of 777. I find the powerbelts require a low power load for no blowby. Then work your way up. I doubt, you will find the most accurate past 90gr of either.

I suggest a different projectile. I have tried almost all of them, and find I always go back to the 240gr .45" XTP mag. Second choice is 250gr barnes Expander.

Been working with 220gr PRbullets, with great luck with a mag subbase and 3-777 pellets. But still working out the details.