I don't think there is a "diagram" for this. Just make a "T". The top of the "T" is something flat across the top of the horns. Then measure from the middle of the "T" straight down to the top of the bucks skull plate. That is the "height" of the rack on the deers head.
However, for scoring purposes, there is no "height" measurement. The measurement used is tine length. It means more to me to hear that the buck had "20 inch G-2's" then "He is 25" tall", but to each there own.
Hope this helps.