First you need a quick way to measure a relatively long length of rope. I measured my "wingspan", distance fingertip to finger tip with both arms stretched out from the shoulder outward, 90 degrees to the body. My wingspan is 5.5 feet. To measure from your high point in a climb, lower the tail of the rope until it touches the ground. You might tie a throw weight to the end so you can tell more easily that you've touched the ground with it. Now grab the tail at the height you are and measure wingspans as you pull the tail up. Easy way to calculate is count how many wingspans it took to get the end up the rope up to you and multiply it by your wingspan measurement. This is an approximate measurement of your height in the tree.
If you want more exact measurements pick up a laser rangefinder, Nikon makes some nice ones, they are relatively inexpensive for what they can do. For example you can measure tree height from the ground etc. Also nice if you're making a rope zip line and you want to calculate how long a rope you'll need to go from point A to point B.
The following user(s) said Thank You: oldtimer, tmaclean