Wear a hat. Mark trees at ten yard intervals in a rough line. Look at the animal and lower the brim of your hat until thetopmost thing that you can see iswhere you intend to shoot. Turn your head without moving it upor down until you are looking at the line of trees. If the brim of your hat lets you see at the top of your vision the 10 yard marker, you are 10 yards away, same with 20, 30, or anything in between.

Simplest method ever.