... 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. ...
But of course some of us can't leave it that simple. Lately I've been subtracting the average size of the two holes I'm measuring from, since the holes are rarely the exact diameter of the bullets. They can vary from one type of paper to another and even from one hole to another on some paper. Now that I've finally gotten under .5 MOA it can make a noticeable difference. And I use calipers now too, so I can get 100ths of an inch, or 1000ths if I ever feel the need. Probably won't need the 1000ths this year or next.
But digital calipers sure make short work of it.