If you are buying protection for the hind legs, the measurement is normally taken on the front limb. It is in fact very uncommon that between front and rear legs, there is such a difference to require boots of different sizes.

Many people think that the protection must cover the entire deep flexor tendon even up to the carpus (the knee). Not only the part of the tendon most exposed to trauma is the lowest, but a protection that is too high would just bother the tendon when flexing, particularly during the jump.