Subtract bullet diameter is very optimistic - none of your holes will be that big. It's *much* more accurate to measure from the outside of one hole to the inside of the opposite hole.
Now that I've examined the question a little more carefully, I can see that you are right.
I have a number of bragging targets cut out and pinned to a bulletin board, and I took a closer look at them. I had always assumed and believed that the carbon marks made by a bullet passing through a paper target were exactly equal to bullet diameter. I took some of these target cutouts down and measured them. The carbon marks appear to be just a little less than bullet diameter...not enough to make a huge difference but enough to make the outside-to-outside measurement minus bullet diameter a little optimistic. The inside-to-outside measurement would be much closer to exact.
Yeah, it matters a lot more with a ~ <1/2" group than it does with a 2"+ group!
