This may not be what you want to do but it is similiar to the one that I had seen before. This way you wouldn't have to go out an by a new sight.

http://www.bullseyearcheryrangefinder.com/how.htm

This is the one I was talking about at first.

http://www.deadonrangefinder.com/howitworks.html

Just a suggestion in case you don't want to buy a new sight.

ty