The bicycle geometry 101 (http://www.os2.dhs.org/~john/Bicycle_Geometry/) spreadsheet has the compound mitre profile in the last few sheets. I never figured out how to use the rest of that document sadly.

What I do. I have a cad that gives me the length of the overlap between the tubes at the BB. I did the math at some point but I can't remember how hard is was to derive. Anyway once I have that length I scribe it from the BB mitre on the tube centreline, cut a v with the hacksaw, and scoop the mitre out with the proper half-round file. I set the tube in the vise at the proper angle and file horizontally.