I'd start with a load around 25 gr 3F, a patched roundball, and work up towards 40 to 45 gr. Where in that range you settle would depend on accuracy vs. energy obtained or needed for whatever you're shooting at. I'd expect you to get about the same results as the .50 lyman plains pistol, and for it (if memory serves) 40 gr of 3F was good for around 800 fps muzzle velocity.