I have the same shotgun as you, but have only done limited round ball shooting with it. I've also not bothered miking the bores to try to come up with a "better" combo than approximately what has already been described. I just worked with what I have on hand.
I have a supply of .690 balls, and some "fit tests" at the muzzle revealed that some .220 patching I had on hand was about right. Lubed with TOW mink oil tallow and launched over 80 grains of 2f Goex, both barrels print pretty close to the same point of impact at 25 yards. I'm lucky in that I guess, because different POI's for each barrel is more common.
I never pursued "more accurate" loads because I'm just not skilled at aiming without front and rear sights. Someone else could probably do wonders following a bore miking session, adjustment of components and load testing.
I got 3" or so groups from both barrels at 25 yards, but we won't talk about what was happening at 50 yards. In my hands mine is a fer sure 25 yard deer gun, but it would probably make a better club for 50 yard golf shots. :rotf: