The ruler function on google earth is a handy tool to use. Just measure the distance from your stand in approximation to certain landmarks (trees, ponds, ditches, etc.). The trick, though, is that the ballistics information on your box of ammunition is just the general behavior of the bullet, and it may not applicable to all guns so you'll need to get a box of bulk ammo and practice at longer ranges and in a variety of wind conditions. As much as I like to pride myself as a decent shooter from the bench, it's a different ballgame when there's a deer out that far and the adrenaline starts seeping into your system. I have found that if I am going to shoot over 200 yards, I like to be on the ground and shooting with a bipod. Those minor shakes and bad trigger pulls that cause a shot to go one or two minutes from the bullseye at 100 yards REALLY get amplified at 300 yards.