QUOTE ... "As far as how much is too much, measure the front of the lowest batten in your sail from the foot, then see how far up inside the mast your extension goes.
If it extends beyond the height of the lowest batten (the foot batten) then you need a little shorter extension."
Yes, this is a rough guide but if in doubt dont waste your time trying to be a windsurfing engineer .. just read the designer's recommended specs.
Plenty of my North sails have the Xtender connection much higher than the foot batten, and that is as per the OEM spec!
So make it easy .. just read the specs.