Welcome to the forum First, Powerbelts have a terrible terminal performance reputation - no way I’d consider shooting them at anything other than paper. The Boredrivers are a good bullet.
It’s easy to overthink this stuff when you’re new. I’d get yourself a volumetric measure and start with about 90 grains by volume. See what your groups look like and then move up in 5-10 gr increments. The max volume charge in that rifle for Bh209 is 120 gr.
And, the most important thing - use a witness mark on your ramrod to denote “loaded.” This will prevent double loading the powder, bullet or both. (Load the rifle and either use a piece of masking tape or a silver sharpie to mark the spot where your ramrod is at the muzzle.)
What are the other 99 questions?