You can't base it on one group.
Take a 20 shot group that is about 2", you will likely be able to pick 5 shots in a cluster that you could cover with a dime. You know the rifle doesn't shoot that well because you have the rest of the group.
Now suppose those five shots were the first five fired. Sometimes what looks really good is just a statisical abberation. You can do coin flips and get heads or tails 5 times in a row, but over time it will be close to 50-50.
You can't always tell how good a rifle is with one group. I've had some groups around 1/2", when I loaded more of the same load they were 1.25"