I don't use one. Many years ago I learned to use the duplex reticle on my old Leupold for ranging. Since a deer where I do most my hunting is normally 20-22 inches through the chest I played with it through the various power settings to see what power the reticle covered the distance from at different ranges. I can just go through the power settings 3-9 until the chest is within the duplex and it gives me a good range estimate. For instance I know at a 5X setting what range a deer chest falls within the duplex (right at 250 yards) and since I'm already on the deer the shot is taken right then. Once I got used to it, using this system is easy but if I ever change out scopes I'll have to do it all over again.