Wait for Aussie531 to chime in
He's a tall guy who's gone through the same process and come back again. I'm a proponent of proportional length cranks but my interest is at the other end of the scale where short riders are always forced to ride cranks that are too long. Anyway if your basing your desire for longer cranks on the 21.6% of inseam figure then I think thats too long. It MAY work for some but I think its too long and prevents the rider from adopting a good aerodynamic position. I recommend that cranks should be in the RANGE of 19% to 20.5% of inseam. To me the 21.6% of inseam figure is both too restrictive and TOO long.
EDIT: Actually you can search for crank length and look for Aussie531's posts. We've discussed this many times.
Anthony