The loads in Graeme's book are for regulation with open sights. The scope effectively changes the plane of the point of aim since it is now higher at the rear, but I guess you already know that. His book has good diagnostic information on why the groups from each barrel are separated, in regards to the direction of separation.

Trial and error I guess. I haven't seen the setup of the adjustment screws but I figure that adjustment will likely change to point of impact on both barrels - obviously moreso on the left hand barrel but I reckon this will still cause some affect on the right too.