Being a good judge of yardage takes a lot of practice. I found that shooting the field hunter round, where your targets are different sizes at marked yardages sharpens my estimations. Shooting 3-D shoots will also help you become a good at estimating yardage on different size game, but is a bit more difficult because you have no specific mark as on a paper target. Shadows, shooting from brush into open areas or in the snow can still be a challenge. I try to estimate in increments of 20 yards by looking directly at the target and eliminating everything else between me and the target. This usually gives me a fairly accurate estimate.