Willyp,
You should be able to start with 50 grains of triple 7 powder or one 50 grain pellet. Most in-lines can safely handle a maximum 100 grains of triple 7.
I would suggest starting around 65 grains of loose powder and working up slowly in 5 grain increments. Be sure to foul the barrel and breech with a couple fired primers (only) before loading. Then each and every time you reload, run a spit patch down, reverse the patch and you're ready to load the next round. When dumping the powder, turn the gun on it's side, pointing barrel up....hit the side opposite the breach with the palm of your hand to settle the powder. Try to be consistant with the ramrod pressure when seating the sabot/or whatever patch and ball.
Jim
p.s. Fot what it's worth, my best accuracy is with a 295 grain powerbelt and 95 grains of triple 7.