My How You've Grown!

Ever wonder how tall your child will be when they finally stop growing? Researchers at the University of Saskatchewan in Canada have devised a formula that can predict the eventual height of healthy girls ages 8-16 and boys 9-18 to within a couple of inches. You plug in information: gender, birth date, height, weight, and sitting height. Be sure to click on in the article where it says: "Detailed description of the measurement protocols can be found here."
So you are properly doing the measurements as needed.

http://athena.usask.ca/growthutility/phv_ui.cfm?type=2