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It's best to measure while wearing skates. Measure from the center of your knee cap to top of the boot, in proximity to the top skate eyelets.
Short of wearing skates, measure from the center of your knee cap to the center of your ankle socket. If you wear your shin guards tucked behind your skate tongues, then it may be practical to drop down one size in shin guard (if you measure a 15", consider dropping to a 14").