Instructions
The image can be pasted from an URL (web address)
The first two clicks need to correspond to an inch apart on the picture.
For example, you can click either side of a circle that is known to be 1" diameter
or draw a 1" line before photographing the target and click at each end
This tells the programme the correct scale of the target
The third click should be the centre of the target

Then click the centre of each shot. The group centre and extreme spread will be calculated automatically