It is at your eye. That would be closest to the comb. But, error caused by measuring to butt is easily calculated.
Say you measure 36" muzzle to eye on a 48" over all length gun. On a 16 yard target, the ratio of 1"/16 yards = 1/16" / 1 yard. Lets now do it in inches to make the 48" OAL easier to deal with. 16 yards = 48 feet = 576 inches. 1"/576" = (1/16")/36" [1/16 X 1/36 = 1/576]. 1"/576" = (1"/12")/48" [1/12 X 1/48 = 1/576].
So, if muzzle to eye was 36", the gun was 48" muzzle to butt, and the pattern was shot at 16 yards, you would adjust 1/12" at butt for every inch the point of impact (POI) was displaced from point of aim (POA). Keep in mind that adjustments at butt are intended to make adjustments at eye.
Hope that helps. Questions?
