If you're not looking at a "No-Peep" as the answer..........well you SHOULD.
Seriously, I can shoot spot accurate to any range with nothing on my string but a string loop.

What your results are telling you is that you do not have repeatable form yet, and that will only get worse as you start shooting from trees, adding headnets, gloves etc that change your anchor and worse yet, prevent you from accurately feeling your reference points. Think about it.......you are having some difficulty in front of a target, now add in a 20ft height, a headnet and gloves and a moving animal that you are trying to determine range on. Things happen fast and bad form can happen just as quickly.
It can be done, but man does it take discipline.

I hate to sound like a commercial, but the Timberline "No-Peep" is the answer....well at least a very good answer. It's just a quick reference that's right there in your peripheral vision and it's extremely precise to any variation in form and anchor.