How can you estimate the heighth of a child by measure from thier knees to their ankle and consequently multiply by ?

How can you estimate the heighth of a child by measure from thier knees to their ankle and consequently multiply by ?
Sure, but the most accurate way, and the passageway I have other done it is to measure from the floor to the top of the pave the way. When I want to know how tall they are, I don't want an estimate.
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You lug this measurement on the child's second birthday, and next multiply by 3. This is supposed to be an indicator of their full adult height above sea level.