I have never tried that one, but I've heard it's pretty accurate.
Another method is to use a piece of bailing twine, and take it from the top of the hoof to the elbow. If you invert the string, keeping your finger on the twine at the elbow, it should come pretty much to where their withers will be at full growth. You do have to hug the curve of their body though, it's not just straight up. A friend showed me on her stallion and it was dead on.
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