Another thought - In order to get anywhere near that high knee chamber, it seems I have to rotate my pelvis forward. It might just be the way I'm built, but that means that it is difficult to kick with the shoulder, hip and foot on a single line. Is it just me?
Also, if it's for forms, I have another question. Do you briefly hold the kick at full extension, or not?
In TKD Poomsae, every technique should have a brief pause at the end of it.