Five observations and a recommendation from one who isn't much of a paddler, but lives at the fringe of the bell curve vis-a-vis surf ski and wave ski fit:
1) Proportion of lower leg to upper leg makes a diff. Once I had a ski that
had my knees at least two vertical fists off the deck - so high that they
kept getting in the way of my paddle.
One of the windsurfing shops in Hatteras had a guy who was exactly my
height and had *precisely* the same hip bone-to-heel measurement.
But whereas my shins are unusually long relative to my thighs, his were
the opposite: somewhat shorter than most relative to his long thighs.
That ski fit him perfectly.
Being somewhat spatial-relationship retarded, I cannot explain this; but
I can assert that did, indeed, happen.
2) Posture makes a difference. When I go from a slight slouch to the proper
paddling position of relatively-straight spine leaning slightly forward my
hips rotate back and my effective leg length diminishes by about 1.5 inches.
3) Shoes matter. The diff between bare feet and the windsurfing shoes that
I wear when paddling is about 3/8" of an inch.
4) There's a good 2-3 inches diff between what feels comfortable at first
and what I wind up with as my "dialed-in" leg length. Longer at first,
then shorter when dialed in.
5) The opportunities for miscommunication and subtle differences in
measurement methodology are many. You follow somebody's system
exactly, you'll come up with one number. Somebody else follows it
with your bod and they'll come up with another number.
On my "longboard" wave ski, I followed the maker's instructions to the
letter. Went through them 3 times, getting the same number, called the
guy to make sure I was doing it right.... and the ski came in with a good
3 more inches of leg length than I really need.
The recommendation is this: favor an adjustable cockpit and make sure it
will adjust to a good 3" longer than you think it really needs to be - as long as
it will go at least an equal distance the other way.