Factory 3/4 oz. duplication load is published Jon. It's 15 gr of 296/H110 and they all go 1100 fps at 13,100 to 13,500 psi which is where the 3" 410 runs.

Not to worry.

In fact the goal here is low-ish velocity.

I might cut the loads for the left barrel by a grain and dribble in a few more pellets to take up the room.

It's a 410 after all, and retained energy past 35 yards isn't important.

"The price of good shotgunnery is constant practice" - Fred Kimble