Having examined hundreds, if not thousands of doubles, it is obvious that barrels converge. It is also obvious that convergence is not as critical as in rifles, and this can be concluded by the fact that regardless of barrel legth or gauge the barrels touch at the muzzle. In fact you can tell a chopped double by the lack of muzzle to muzzle contact.

As long as the patterns hit to the same point of aim at the required distance there is no need to work out the trigonometry of the thing.

More serious fault in barrel alignment is the vertical deviation as this can be a cause of regular and puzzling misses.