You're right, Ron - regulation is spotty with a lot of guns. Even Forsyth addressed the problem or getting shotguns to regulate with round balls, way back in 1860. He noted the problem was shotguns normally had wider breeches, with more angle to the bores toward the centre, making them cross badly with ball.

Perhaps I was lucky with my double, in that is was fairly slim, that is, the barrels were not really thick at the breech. Mine, with heavy loads for black powder and smokeless, both in the 1,400fps to 1,500fps range, it regulated just fine - round groups at 50 yards in 3" form it's 26" tubes. It would not shoot modern slugs into a consistant 8" at the same range, however.
You never know until you try it.

As far as I know, there is no know perscription that says, do THIS and THIS will happen. You do this and that and hope you arrive at a usable conclusion.

Worse comes to worse, you sight for the accurate bore or barrel, and learn to hold for the other. This was the way of 1860 and works today as well. Several of the guys here played with them with success.

It's a balancing act with wads - small changes in load can make big changes on the target.