No need for math and charts, there is a simpler solution. There is a website called Dry Fire Online http://dryfireonline.com that can do the same. I can connect my laptop to my TV and I can display the targets there. It can have even moving and random targets. Just enter how big your screen is, and how far you stand from the screen and that's all, just add some targets in the stage builder. Then if you place a target at 20 yards, its going to have the same size (from the shooters point of view) like a real target at 20 yards.