I have a round ball formula that seems to actually work. I don't know where it came from but have had it for years. The forrmula is for using GOEX 3-f and PRB. This is a starting load.

caliber X rate of twist X 2

Example: .45 x 28 x 2 = 28gr

As I said, using this formula I would start at 30 gr with your rifle. This works for me on everything from my little .32 to my .58.