I, too, use a Sligo stick.
Measuring is somewhat arbitrary as on any given day at any given time at any given location, a horse will measure differently. Weight, hoof trim, how the horse is standing, etc, all come into play when measuring. I can measure a horse 3 different times and get 3 different measurements. However, when my measurements are within a 1/4" of each other, then I am pretty confident of my horses' height.