That’s pretty much the old fashioned way of doing it, but it still works. Whether you mark it with tape or chalk, you should still get a good estimate. I just saw an ad recently for a meter that clips onto you to measure this.
1) Have your client put a small piece of double sided tape on his or her index finger.
2) have the reach up while standing, place tape on wall
3) put another piece of double sided tape on finger and without running jump, have them jump as high as possible
4) take a tape measure and record the distance between the two pieces of tape on the wall
The distance between is your vertical jump