I know exactly what you are talking about.

There are marks on the ring of the sight. You line the bottom mark up with the belly and whichever tick mark touches the deers back tells you about how far it is from you.

I could see how these things might work alright from the ground, but when you get up in a tree i could see how things could get all messed up.

I would just be safe and get a rangefinder or mark off your distances prior to hunting and tag them with something.