When an image gets out of focus, a point source of light (a small shiny object) becomes a circle of light as shown in the following examples. If you are lucky enough to have a point source in your photo, you can measure the diameter and enter that figure for the "Blur Width".
This medicine bottle in the background of a photo has light reflecting off it. This is a point source which has become a circle when it became out of focus. The image on the right has zoomed into that point source, and shows that the diameter is about 18 (distance between black dots).
Note that we have placed the black dots approximately in the middle of the blurred edges (ie. between the blue dots). To zoom into the image, you need to use another program such as Photoshop or Paint Shop Pro.
You might need to be quite observant in looking for point sources. This image of some trees in the background of a photo has a point source. Can you see it?
Sometimes you can have a point source bonanza as shown here. Here we have two photo's, one taken "out of focus" and one taken shortly afterwards "in focus". They clearly show the "circle effect" for point sources.
Here is another difficult one. There are three point sources in this blurred picture of a car's license plate. Can you see them? The diameter of the point sources determine the Blur Width setting to use in Focus Magic (see Tutorials).