Here's a guide for shot loads:
POWDER MEASURE SETTINGS TO
MEASURE OUT OUNCES OF BIRD SHOT
50 grain setting = 3/4 ounce of shot
60 grain setting = 7/8 ounce of shot
70 grain setting = 1 once of shot
80 grain setting = 1 1/8 ounce of shot
90 grain setting = 1 1/4 ounce of shot
100 grain setting = 1 3/8 ounce of shot
110 grain setting = 1 1/2 ounce of shot
120 grain setting = 1 5/8 ounce of shot
Generally, the rule of thumb is to use an equal amount of powder and shot. But in reality, people use a little less powder than shot depending on the type of shooting, whether hunting or target.
I'd suggest a load of 80 - 90 grains of powder with about 90 - 100 grains of shot. Some folks may go as low as 70-75 grains of powder with a little less than 90 - 100 grains shot, but muzzle loading shotguns tend to shoot wider, more open patterns. So more shot usually gets better coverage, and then tweek the powder upward to improve velocity, range and lethal performance.
90-100 grains of powder with 100 -110 grains of shot (by volume) should be a very effective and potent load.
It helps to use an over powder wad & an over shot card, and try to make sure that the load in the 2nd barrel stays on top of the powder charge. Keeping the muzzle elevated between shots will help to accomplish this if the load is packed too loosely.
I've substituted newspaper & cotton balls for wads when I didn't have anything else to use, and modern plastic wads can improve patterns dramatically too. But I would still try to protect the plastic base with some wadded newspaper or over powder wads and over shot cards. Some people have reported some soft or melted plastic, but I didn't when using Pyrodex and enough newspaper wadding under the modern plastic wad.
But be careful about smoldering embers exiting the barrel if using newspaper or cotton ball wadding, especially if there's any fire hazards where you shoot.
Good luck & let us know how it shoots.