[Greg McKaskle]

The distance calculation in the example uses the width of the blob as a proportion of the image size. If you scroll down on the diagram, it has some pictures and formulae to show what it is doing. By the way, this estimator works better for some field positions than for others. It is also possible to use the height of the blob, the area of the blob, or use tools like the clamps to measure things more precisely than the bounding box.

Tutorial 8 in the Getting Started also discusses this a bit, but I don't remember how much detail it goes into.

If you have other questions, I am happy to help.
Greg McKaskle