Bob, I am given to understand that it is more than a math problem, the effect of the shooters body and "hold" affects the point of impact and is very difficult calculate. Granted, this is likely easier to see with a double rifle than a shotgun, but it is still there. Milling the rear part of the top rib is often done as you described. One thing I recall and didn't mention is when drilling the holes for the barrels in the monoblock, they would leave one of them smaller than the other. This is to give them the ability to move it slightly, if final measurements show movement is necessary.
Mike