I used to suck at judging distances - a lot of stump shooting has cured me. I don't ascribe a number to a distance during the shot sequence, but I do have a 'circle' in my head for when an animal crosses my maximum range - once I know that it's within 20 yards, I set up for the shot, and let the internal computer take over.
Your subconscious aiming during instinctive shooting does the estimate for you - not in actual numbers. But first I believe you have to teach your conscious brain to do the estimates, until enough shots makes it 'instinctive'. Instinctive just means that you've done something enough that you don't have to think about it anymore.