Since it can be difficult to find the center of a hole, you can measure outside to outside and subtract one caliber.
This is ok for relatively large groups, but when the group size shrinks down close to the caliber size, the relative error of this method becomes large.
I've never seen a rifle bullet hole in paper that was the size of the nominal caliber. The holes are almost always a good bit smaller than what one might expect.
In spite of the fact that "eyeballing" is required to guess at the bullet hole center when measuring center to center, the "eyeballing the center of the hole" method always gives a more accurate number than measuring outside to outside and subtracting caliber.
As an example, the last time I went to the range, I was fortunate enough to shoot a pretty good group at 50 yards with my CZ-452.
Measuring center to center, I consistently get 0.29" BUT, if I measure from outside edge to outside edge I get 0.405" and when I subtract .224" (nominal bullet diameter) and then round to 2 digits, that gives me a group size of 0.18"
That's about HALF the actual size of the group. Where does the error come from? I measured one of the holes shot on identical paper and with the identical ammunition and rifle--and it's NOT 0.224" in diameter. In fact, it measures ONLY 0.12" in diameter. That's about half what one might expect based on the bullet size.
If I measure outside to outside I get 0.405" as before. Then when I subtract 0.12" (the
actual hole size) from that number and round, I get 0.29" which is the same as the center to center measurement even though I got that measurement by "eyeballing" the center of the holes.
If you're going to measure outside to outside, you need to subtract the ACTUAL hole size as measured on the paper, NOT the nominal bullet diameter. Otherwise you're almost always underestimating the size of your group--perhaps by as much as half the bullet diameter.