Yes, outside to outside minus the bullet diameter is the way I was taught. The strangest match I ever went to we were shooting groups with muzzleloaders. Two RO's pulled a string to the centers and measured the string. I offered a set of calipers, should have seen the look they gave me!!!!!
Or there was a time in an indoor range, a guy in the next bay ran a full size human silly-wet out to 10 yds, and proceeded to put 10 shots through the x ring with a 223 ar. replaced that target with another, same drill. Did that 6 or 8 times. Neatly rolled up all the shot targets and left.
Here's the question, were those targets shown for group size at x? yds or for his ability at long range. We will never know.
Tony