Forget about a rangefinder. Limit yourself to 20 yards or less. It's either in range, or it's not, and only one pin to worry about. Put some markers out if you have to to show a 20 yard radius around your tree. I have killed more deer under 20 yards than past 20 yards and I have killed more thana few, so don't think you are severely limiting yourself that way. Once you punch one right, it all gets easier from there.