Practice estimating distance in 10 yard increments and you can come pretty close .
Pace off a 10 yard distance and remember how many steps it takes . When you figure out what tree you will be in , pace off the distance to a couple of other trees , etc for reference . When a deer shows up , it will give you a general idea of how far it is .......
If you go the Rangefinder route , remember , it's the horizontal distance that counts .