Did your rifle come with the paperwork? If not you ought to call them.
I'm assuming it is a sidelock/flintlock rifle that probably has a max load of 100-120 grns of powder.
Here's the max load for a Lyman's rifle. Though different it ought to be similar.
.535" RB 120 grs. 2Fg or 100 grs. 3Fg
335 gr. Sabot 110 grs. 2Fg or 90 grs. 3Fg
450 gr. Maxi 110 grs. 2Fg or 90 grs. 3Fg
The powders don't weigh the same, but weighing your charge can get you more consistent. I.e. fill your measure to volume you want (80 grns), which, for the sake of argument, weighs 71.2 grns. You know that 80 grns volume weighs 71.2 grns weight of your specific powder, and can weigh your charges if you want to be more precise.