Measuring from the top of the femur may make more sense for the scientific types but its VERY hard to measure properly if your a mere mortal. I recommend measuring your inseam from hard against the pelvic bone to the floor in no more than socks. Then based on my experience of having tried MANY different crank lengths and the experience of others I recommend cranks to be in a range from 19% to 20.5% of inseam. There isn't just one length that is perfect and all the others are wrong. A 5 mm difference is nothing assuming both lengths are within the my recommended range and even a 10mm difference isn't that great as long as the bike is setup properly for the different lengths.

Anthony