Regarding the number of shots per group that's going to give good data, consider the following.
Here's a plot of 250 shots. Do the following to get a few 5-shot or 10 shot groups:
1. Pick 5 (or 10) shot locations, then plot them on paper.
2. Pick another 5 (or 10) then plot them on the same paper but keep them placed relative to the same center point of the 250-shot group.
3. Repeat step 2 a few more times.
4. Now look at your several few-shot group composite.
5. Find the center of your composite.
6. Where's that composite center relative to the 250-shot group center?
This is the best thing I've ever seen to show how important it is to shoot enough shots per test group to be meaningful.
Note the shot spots you've not picked will be shot later should you "shoot" all 250 rounds.