Postman,

I think you are on the wrong track here. Regulation of a double rifle involves finding a common point of impact for the left and the right barrel at a specific range. The key here is that each barrel recoils differently, what looks good at 100 yards rods won't look good at 50 yards as the trajectories for the two barrels can only intersect at one point. You are better off focusing on the yardage you are interested in.