It's been years since I've had one. It doesn't take as much as you would think. If you have an open area to fire it, start with around 90 gr., and go up from there. You can increase the load, watching what the projectile does. It will go from blooping out of the tube, to doing a big launch. No sense using more than necessary.
FYI, if you launch a full beer, they tend to be a bit shook up when you open them!