What works in muzzleloaders also works well for shotshells. I shoot a lot of muzzleloading trap events and one of my pet loads for a 12 gauge shotshells is 80 grains of FFg, a 1/8 inch thick card wad, a 1/2 inch fiber wad, and 1 1/8 ounce of shot.
This won't completely use up the space in a brass shell so you might want to use extra card wads if filling up the shell entirely is important for crimping purposes. Since I use a single shot for BPcartridge trap, I don't crimp or glue the overshot wad, friction holds it in place just fine.
If useing BP substitutes, that's 80 grains measured by a volumetric powder measure calibrated for black powder, not 80 grains actual weight. If you don't have a BP powder measure, an empty .30-06 case will measure about 70 grains. An empty .44 mag case holds about 40 grains so two empty .44 mag cases full of black powder would approximate my load.