I can't vouch for the accuracy of the method you've inquired about, but many do feel that it is 'reasonably' dependable....
You need to measure while the foal is fairly young, but after it has at least a few days to 'unfurl' after birth, IMO. Most accurate will likely be a dressmaker's (soft) tape. The horse should be standing up straight on all four. Find the center of the knee joint, measure straight down(don't try to follow the 'contour' of the front of the leg; measure 'straight') to the coronet(where the hair meets the 'top edge' of the hoof ). To 'predict' adult height, you multiply that measure by four, add 2"(some say one inch, but 2" is what I go by, and I believe, is the most commonly-used figure.)
You aren't ACTUALLY measuring 'the cannon bone', you are measuring the cannon PLUS the whole of the pastern and fetlockand the bones that make them up. The cannon itself is thought to be as long as it will EVER be at birth, although the 'ends' of it will solidify and 'grow' until maturity--even so, that is why it is thought to be the one to use for this 'prediction'.
Good luck with your foal.
Margo