All of these ideas and calculations are correct as are none of them! That is why the only solution is the tried and true, trial and error method. Every rifle is a bit different, hence every rifle will have a slightly different P.O.I. This is true even of rifles (and shotguns!) of the same make, weight, etc. Sure you can make some calculation maybe to get you close but it'd probably be just as easy to use a BIG target for those first shots and make your calculations for the next soldering according to where the initial P.O.I.'s fall on that large sheet of paper.

That said I do find the idea of assembling some data base of KNOWN convergence degrees (breech to muzzle) interesting, as said simply as a reference in the initial assembly. However, if this truely was useful I'm sure it'd be used by the established makers for many years and someone would know something of it. As yet that doesn't seem the case eh?

At any rate, given the very low numbers of such rifles to be produced by the home craftsman such an approach seems hardly worth the potential benefit. Lacking that is one of those fairies!