I hit a British penny at 300 yards one time with a single shot.
'course I'm not going to tell you how many other "single shots" came before it, or whether or not I had given up on the penny and started shooting at something else by then, or the 8-inch halo of "leaded paper" around the penny. Unless there are people around who actually witnessed the event, and that number is dwindling far too slowly, I usually just go with the first part of the story.
Anyway, I usually measure with a coin of various description, or a flattened bottle cap. If the object covers half or more of the hole, I call it in the group. I have a bit of difficulty imagining a 1" group at 75 yards or whatever measure you want to go with. I prefer an object for reference.
When I was a scout, we would shoot something like 30 rounds, and then the instructor would walk out there with a quarter to measure groups. He would tape over every 3-shot group he could make with a quarter. I always kinda thought that was just a bit on the cheating side, but I suppose it works for boy scouts.