Have you looked at or talked to the people at Rocky Mount. They list 3” .410 shells. I’d check their internal volume as a starting point. See what their largest shot payload they recommend. I know you are looking for that 3/4 to 7/8 ounce .410 payload. In the end I’d rather have a higher velocity 5/8 or 11/16 ounce, tighter pattern load, than a heavier, slower load. Trade off in all things.