Ahhhhh - a Tige breech! The correct charge is whatever it takes to just barely to the top of the tige(peg). A "ball", possible conical in shape, & very close to bore size is dropped down the bore and the rod it 'thrown" 3 times upon it to expand the "ball" into the rifling.
A patched ball with 70gr. charge would be my load, unless it shot more accurately with even more powder.
I just goggled Tige Breech and came up with this diagram which suits.

https://www.historicalfirearms.info/post...ned-by-a-french

Happy New Year Tom.