I may be wrong, it has been awhile, but I am pretty sure it is by the 3rd birthday, not the calendar year for minis in the A&R reg.. As for measuring, I honestly don't think that the fluff from a winter coat will make a measurable difference in their height. Regardless of summer or winter coat, a properly set up horse with a hoof in good trimmed condition and placing the horizontal arm on a measuring stick level and firmly in place (no excessive force, just firmly in place at the base of the mane) will dictate a more accurate measurement than concerning yourself with the winter coat.