Quite a few years ago a hunting magazine had an article in it about estimating range using the top of the duplex post and the center of the crosshairs to estimate range in the manner that Flags uses. It said that a lot of scopes, when at their highest power, would have 6 inches between the post top and crosshairs at 100 yards. At 200 yards it would be 12 inches, at 300 yards it would be 18 inches. If a deer's body at broadside is 18 inches from bottom to top then we have a system. If the deer's body more then filled the space then it is closer then 300 yards, if it does not fill the space then it is further away. If it only filled 3/4 of the space it was appx. 400 yards away. You get the idea. Of course you need to check your scope at the highest power at 100 yards to make sure that it has 6 inches between post top and crosshairs and you need to know where your rifle shoots at the various ranges. I use the system with a 30-06 sighted in at 200 yards, at 300 yards it shoots a few inches low, at 400 yards about 2 feet low. I don't usually shoot that far, 300 yards is about as far as I would try and then at a standing broadside from a solid rest.