I measured your group hoc9sw, but I'm not sure it's exactly correct. Here's what I got:
The way the software works is through using actual bullet diameter. This way it can calculate the center-to-center distance between the two furthest holes (represented by the black straight line between two of the drawn holes). The .45ACP is actually .451" or .452" in diameter. The actual measurement of bullet diameter isn't critical as long as you're able to put the virtual hole centered around the actual hole.
In order for the software to do this, it must be calibrated for each picture. This is done using a known reference in the picture itself. hoc9sw has given us the relevant reference.
As you can see, the largest distance is not always obvious. I would have thought the top most hole and the bottom most hole would have made the group. In this group though, those aren't the largest distance.
The challenge in this particular target is how the holes were made. I'll bet he used hardball ammo and didn't have a backer behind the target. This makes for lots of tearing. It's difficult to place the holes correctly on such a target. I did my best to get them centered over the holes, but may not have got it right.
Also, the type of paper makes a big difference. Notice the clean holes in DavidR's target just a couple posts above. He's using a different type of paper and what looks like wad cutter bullets. Those rounds are great for both target competition and self-defense. Very accurate and transfer energy almost as well as hollow points.
Anyone that wants to try this software for themselves can get it here:
On Target Precision Calculator There is a free version, which is what I've been using, and a much better one for about $12 (I'm just really cheap). It really is good software and will give you the actual results rather than guessing. It's also very easy to use. If I can do it, you can do it.