If you want a simple way to ballpark your groups for the purpose making better reloads, use a dial capiler and measure the largest distance to to the outside of the group and subtract out the diameter of the bullet. I sometimes like to draw a line through the center of the group and measure across that. Problem is that the bullet hole is always smaller than that of the bullet. As long as you stay consistent you can learn what you need to improve groups when optimizing a load.

When I first started shooting Benchrest, my group in matches measured 0.08-0.100 larger than what I have measured with my method.

In terms of the bug hole software, you have to calibrate on a reference to get it to give good results. I have measured 20 plus groups I shot at matches that were officially scored and compared to what I got on bug hole. I typically was within a few thousands. Official targets have exact size boxes that you can use as a reference to calibrate with. That said, I don't find it would the effort anymore.

Luck, Tim