I sold a colt several years ago, dam is about 35.5 sire was 31.25, sire had a small background and dam's parents were around 34. The colt was almost a year old when sold, and was about 31-32. I knew he would be over 34. Found out he is 40" !!!!! It is a horse facility he is at, but I know they are aware of measuring minis at the last hair, even if they are a little off, that is still enormous from those parents! There are measurements taken from the center of the knee to the coronary band then multiply by four, must be done in inches, so if you have an 8 inch cannon, multiply by four, you get 32" give or take an inch, it is usually pretty accurate, but not fool proof.