Have you heard of the stick trick for estimating where a tree will fall? Take a straight stick and grab the far end, put the near end right next to your eye. Then hold it parralel to the ground, pointing the end where you plan to put your notch to cut the tree down. Then rotate the stick 90 degrees, so it is now vertical. Now move forwards or backwards until the top of the stick is even with the top of the tree. Your standing where the top of the tree will land.
Your creating a 45 degree angle with the stick, so you have an equilateral triangle, where the tree is one side, and the ground is the other. The distance on the ground from you to the tree trunk is how tall the tree is.