Gee, it’s hard to add to all the helpful information you have received so far, but let me try.
The rifle was probably made by Jack Garner, an early Dixie Gun Works employee who has been out on his own for maybe twenty-five or thirty years and, as far as I know, is still around.
The general rule of thumb for a starting load is one grain (weight) of powder for each caliber. In the case of a .50 caliber that would be fifty grains. Personally, I see no reason to go higher than eighty grains tops in a fifty but I know other do go a lot higher. Ball size will probably be .490 or .495 depending on patch thickness.
A few regulars here will probably be making disparaging remarks about your rifle because, while it make look nice to the 98% of us who are ignorant and unschooled, it is not a museum quality reproduction of a documented original.
Don’t let it bother you. Just learn to shoot it, shoot it well, and clean a few expert’s clocks when you shoot against them.