I assume you're referring to the "Hunterbolt".
The previous owner of my gun used only T7 pellets, and shot 3 in the gun. I plan to shoot it for the first time next week, but I'm looking at 2 pellets.
Before the advent of in-lines, the "rule-of-thumb" used to be 1 grain of black powder per calibre, not to exceed 2 grains per calibre. I used to shoot 71 grains of 2F in my .45 calibre sidelock. My guess is that something between 50 and 100 grains of BP in your hunterbolt would be a safe and acceptable load. I'd play with loads in that range 'till I'm satisfied with accuracy and velocity.