It is certainly true that according to the rulebooks of both AMHA and AMHR, American miniature horses are to be measured for height at the last hairs of the mane, NOT at the top of the withers(as horses of all sizes are measured elsewhere around the world....) If indeed the question was only about "how"to properly measure: In either case, for the measurement to be as accurate as possible, the horse must be standing square and in a 'natural' posture-meaning the head/neck not held up nor down-on a provably LEVEL surface. For a withers measurement, the crossbar of a rigid,non-bending measuring 'stick' should be placed on the highest point of the withers(since the withers is a bony 'process', this is not hard to determine, and when level, the measurement read. For a 'last hairs of the mane'-which means, where the last hair emerges from the skin, not where the tip of the last hair reaches to!-measurement, the same basic set up applies, then the crossbar should be placed over the last hairs, and when determined level, the measurement read. Even if(as I say below), you are conscientious in your procedure, however, the 'last hair' measurement is needlessly subjective-'pretending' that a horse is smaller than it is byaccepted height measurement standards is silly, to put it kindly--yet it continues(heavy sigh...)
I study this a lot; I believe that it IS possible, with due care, to determine the 'last hairs' of the mane, even if a strip of body hair has been left(which is common procedure--some aren't skillful enough at clipping, or won't take the necessary time/care, to clip' right up to' the ending of the mane, others are very deliberately trying to 'confuse the issue', to put it politely....)-but, your fingers and your eyes, if you wish them to, will tell you which is which. I agree with rabbitsfizz on EVERY point in her post re: wither VS 'last hair of the mane' measurement---except, on miniature horses, I see no way it would EVER "knock two inches off of the height"! With most minis, even 'maximum' R horses, the maximum difference is likely to be around ONE inch-many will be even less than that, a FEW will be more, perhaps rarely up to 1 1/2"-and that would take a horse with PROMINENT withers combined with a mane that extends further down the back than most!!
It is my opinion that the American miniature horse breed registries don't come off well in the eyes of the rest of the horse world for their continued insistence on the 'last hair' measurement; unfortunately, the pretense seems well-entrenched in the power structure, so that there seems little chance of changing the measuring procedure to match that of the ENTIRE REST OF THE HORSE WORLD....! (So what if my now-34" horse is in truth a 34 3/4" horse? It would be relatively simple to alter the Bylaws to reflect the most common difference -the horses wouldn't "BE" one bit bigger!!)
Margo