I would e-mail David Pedersolli and get the limits of that rifle and what it can handle. In the mean time I would be very mild with any charges. The old rule of thumb used to be one grain for each caliber. I do not know if I would even go that much. There is a forum where they have a lot of knowledge on this kind of rifle and caliber
http://www.muzzleloadingforum.comyou might want to go over there and ask. I am sure they could help you out...