A good starting place is 40 gr (by volume) of FFFg real black powder, or 35 gr (by volume) of FFFg Hodgdon's 777, or 40 gr (by volume) of Pyrodex P. You can use FFg or Pyrodex RS without harm if the others are not available. Use a .490 round ball and a single lubed patch (round or square) of 0.010 or 0.012 thick cotton (NO synthetics) material. Thicker patches can be used but will be harder to load.
Max load in one of those guns is 50 gr. The gun will obviously hold more, but after 40 gr you're pretty much expelling the powder rather than burning it, depending on how it was compressed. The result of overloading is really to decrease accuracy without significantly increasing energy on target.
Max range in my experience is about 50 yards, mostly due to the rudimentary sights that are common on those types of guns. The 40-50 gr powder load is actually effective up to about 100 yards given good shot placement, but the sights make that very difficult.