Grampa000 said:Spliced,
I think you're on to something with the THD concept. Clearly, that is different for different people and is the largest part of what accounts for the need for different sized cues. But you said it shouldn't change much for different sized cues. I say it wouldn't ever change. That's because rather than measuring it as you normally grip the cue, which may not be optimal, and may change a little from shot to shot, the measurement should be taken with the grip arm in the vertical orientation, with the cue level.
You have the right idea but the THD does vary depending on the shot. It gets longer the lower your stance. On a shot where you are able to get very low, with your back arm vertical, your THD may be 50 inches. Now on a shot where you are more upright for whatever reason, your back arm will need to move foward on the handle to be vertical, and your THD may become 45 inches.
It also changes with how far your bridging arm is extended. This varies between shots as well.
When determining proper cue length, the longest THD measurement (when you are down with your chin on the cue, your back arm vertical, and with your bridge arm fully extended) should be used.
Ian