Schadenfreude, I get the impression you are looking for a definitive measure. At a defensive pistol class, our instructor told us we should strive to be able to put all our shots into a pattern that can be covered by our fist. That was our measuring stick at all distances that we shot, from arm's length to 50 feet. Slow fire or rapid fire. Strong hand or weak hand. One hand or two. For defensive shooting, I think that is a very good objective. For bullseye shooting, that would suck. But, I'm not a bullseye shooter! It may sound easy. I don't think it's easy. Give it a try. When you can put five shots into a group that size, try to do it faster or weak hand only.
For defensive shooting purposes, use a target with no discernable bullseye. You are shooting center of mass or head shots. Try on the move, behind cover, etc. Then, when you approach the target, take a look at your grouping. Can you cover it with your fist?
There are lots of IDPA-style drills out there. Try 'em on your target. Then, take a look at your grouping. If you can cover it with your fist, you are a good shot in my book.