Sounds like a modern shotgun with 3 1/2 " chamber for modern smokeless loads. Meaning with BP the gun can handle anything you're willing to shoot!
Back to your question. The old formula was a balanced load, equal volumes of powder, wand and shot. You can do a little experimenting to visually find how much powder fills 1/3 of your hull, same for shot & wad. Then you have the max. Try it and taper down to suit your comfort as needed.
Somebody will come along and tell you what those numbers are. I have not used the full 3 1/2" hull so I don't have the exact numbers.
I knew a guy who shot 200 gr in his 10 ga. I tried one, then stuck to trying his 180 gr loads!