[barn34]

depends what you mean by 'distance' (which side of the target tracking triangle are you looking for). also, depends on what values you're calculating from the image and how you're calculating the angle to target.

the most general way to explain it is that you know how wide the target is (20 inches). you also know how wide the resolution is for the camera (that depends on the camera you are using and how it's set up). same with the field of view, as that's camera dependent.

you can convert how wide the target is in pixels in your camera image to a distance to that target in inches by doing the trigonometry and using the pixels to inches conversion by keeping in mind that the target is 20 inches wide.

that's probably a bit too ambiguous of an answer, but the specifics depend on how you're initially processing the image and calculating that angle to target.