Sak:
I didn't mean that only the last inch is adjusted during regulation (!). I meant that only the direction of the last inch of barrel can direct the projectile on its path. In order to get the last inch 'pointing right', at least 4 to 6 inches of rib should be unsoldered, probably more for anything but the minor final touches.

Ellis Brown's "Rule of Thumb" card came with his Good Ol' Boys Chinwag DVD, but I believe it is included in the new edition of his book. Someone should get his permission to post it here. If you follow his recipe for converting a shottie, weld the quarter-rib and loop the same distance from the breech, choose the same barrel-length, and are kissed on the dick by a fairy, you will probably find the measurements to be a good starting point.