My recollection may be faulty but I believe the rule of thumb for convergence is a 50% overlap in patterns at 40 yards. Ideally the barrels of a SxS will be on the same plane but this rule gives quite a bit of leeway as the second barrel can be anyplace around the first provided the patterns overlap.
As an example, the right barrel may shoot to the mark at 40 yards and the left barrel would be within regulation if it put its pattern to the right and 9" high provided the overlap met the standard. So, regulation can get a bit tricky to understand and is a reason Bruce Buck is not totally off his rocker.
As for rifles, with the large bores it doesn't matter as long as the two barrels get within a couple inches at 60 yards or something like that. Considering the target size that was considered good enough and generally was. I don't think there was any concerted attempt at getting the barrels to cross over at any given distance, it was just a happenstance of that gun.
A guy I knew that collected and shot small bore double guns claimed proper regulation of them had the bullets ideally running parallel with each other. This allowed one to shoot at longer ranges which makes sense as many of his guns were scoped or at least had scope mounts. I still would not think of such a rifle as a precision instrument but it would probably do as far as most would be willing to shoot suitable game.
