Shotguns were never held to very tight standards as far as size goes, In fact, a 24 bore can actually be anything between .530 and .555 depending on who made it.
Also some guns have chokes and others don't. Chokes can be jugged or constricted, so this issue can be quite complex.

But looking at your idea on how to start,--- I’d say you are probably safe.

Remember if you are not getting the results you’d like that about 80% of the time it’s going to be related to patch thickness. A thicker patch and smaller ball cures a lot of problems in these kinds of guns.