I prefer to start with tested loads from a manual. I probably won't even have the S&W .500 Magnum until sometime in the Fall. There is a huge backlog of orders. The .454 Casull and .45-70 can both lob a 300 gr bullet. The Casull can tolerate up to 60,000 CUP and can push that 300 gr bullet at 1600 fps. My .45-70 BFR revolver is only rated to 31,000 PSI by the factory. Much lower pressure. Lower muzzle velocities too. Until I know the specs of the new S&W, I won't be taking unnecessary risks by guessing.
The Performance Center model weighs in at 82 ounces. I expect the felt recoil to be more pleasant than my lighter Ruger .454 Casull at 58 ounces when firing a similar cartridge.
Does anyone really think that the B and F in BFR stands for Best and Finest? :)