Since you don't want to shoot roundball... I'd get some maxiball and start it with 70 grains of Black Powder 2f with some 4f in the pan. If that don't shoot well kick the charge up around 90 grains.

Powerbelts might also shoot well out of the rifle. Or you could try 70 grains of 3f black powder and some 460 grain Bull Shop conicals if the manual allows something that big. I personally shoot roundball out of my Traditions.