The easiest way to get a center to center measurement, which is correct Art, is to measure the group at it greatest dimension and subtract from that measure one bullet diameter.
So a group that measures 2.378" overall (outside to outside) shot at 200 yards with a 7mm would be exactly 1 MOA or 2.094" center center of the farthest apart shots. (2.378 - .284 = 2.094)
Art, it is true that the convention for rifles is 5 shots, but I really believe that a 10 shot group is much more revealing about the true character of the load.
A twenty shot group is even better but you have to draw the line somewhere.
Ni ellegimit carborundum esse!
Yours In Marksmanship