Below is a very good program to measure group sizes by scanning a target into your computer.

http://www.ontargetshooting.com/

I love that software. It is easily as accurate as I am with calipers and more than likely is MORE accurate than I am. It also provides much more information than I care to calculate.

This is a cool program. Here is a pic using a test target I had scanned in.

You can see the hand written measurements I obtained using a dial caliper and "eyeballing" the extreme points.