Thanks Andrew (and Tehllama)!
Sounds like I should try a few bikes with a variety of different reaches to see what feels good and work on finding what that usable range of reach is for me. And if at 5'9", you find a reach 450-480+ works best = sounds like the Instinct with a 454mm reach is gonna be too small for me.
And yes, I'd gleaned that the ETT combined with reach is an important measure to note as the seated cockpit is very important (where you spend most of your time, climbing). Essentially - ETT for seated reach, Reach for standing reach.
Did you mean "The slacker the real HTA the shorter the Reach?" or "The slacker (shorter) the real ETT the shorter the Reach?"
Do you find there is a much narrower range of seated cockpit reach that works for you than the standing reach measure? Ie. Do you adjust your cockpits (saddle position/stem/bar) so the seated reach is within only a few mm of each other?