Certainly that is one of the best solutions, as you are able to combine what "should" be right with actual results in both efficiency and comfort. I would certainly say it would be worth the money to take advantage of such a service if available but would assume there are very few shops that have that type of equipment and service. I don't see how the crank length formula is terribly helpful though, as that corresponds to about a 5mm difference between .21 and .216.