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.
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