Andreas,
I have done a little bit of double rifle regulating and have never really found any way to calculate the amount of convergence for different double rifles.
If you were to build several very similar doubles in style and cartridge, you most likely would be able to come up with a usefull measurement.

For me, I start at 100 thou convergence every time on each rifle and work from there.
100 thou might be somewhere close to a good starting point, but it could also be a long, long way off as well.
I have never really worried too much about it 'till I shoot it and see what needs to be done.
I've always used the old standard wedge between the barrels for regulating as I have personally seen some other styles which were not always successful.
I make my wedge plenty long enough, so that I can pull it out some distance if the barrels start out shootin apart by a long way, and then there will still be plenty of wedge left for a strong joint.
A long wedge means I can also drift it in a good distance if the barrels start out crossing by a long way.
Sometimes the 100 thou could be so far out that a new wedge needs to be made.
Sometimes 100 thou is pretty close to make a good starting point.
100 thou is approx 2.5 mm.
Much depends on how much barrel length is being moved during the regulating.

The point I'm making is that I like to shoot the rifle and see how the barrels print on paper.
I start at 20 yards, bring the two groups roughly together at that distance, and then work out to longer distance and do the fine adjustment at the wedge.
There are quite a few tricks that you learn along the way, especially with the difficult ones to work on.

There has never been a regulating job that I didn't totally enjoy doing, even though a couple were very hard to get right.

Edit
Thought I'd better clarify that the 100 thou I'm talking about means that the bore centers are 100 thou closer togther at the muzzels than at the breech.