I think the theory behind it is that the cannon bones stop growing in length by 6 months. That is why it doesn't matter if it is 6 months or a year. In this string test, you are only measuring from mid-knee to cornet band.
I think there is a string test where you measure from elbow down, and then swing it up over their withers, but I can see how in that one you would need to wait until they are a year, because the upper arm bone (I don't remember what it's called) doesn't close until later. But I have heard that test isn't as accurate as the knee-down one.
So I don't know if it is right, but I thought it was kind of exciting. If he never gets past 15.1 that is cool too, it will make him easier to mount.
But I am kind of thinking he will be taller than that, because his knee to cornet measurement is about an inch longer than his moms. So at the very least, he should be an inch taller than his mom I would think.
All of this really isn't important, I just think it's kind of fun to speculate. And I wrote his measurements down, so maybe a couple years from now I can see how he measures up!
I wish I knew how tall he was now. I don't have a proper measuring stick, which I think is the best method. I have a weight tape with "hands" on the off side. I will get a guesstimate with that, but I have a hard time eye-balling it that way.
I would love to get a proper measuring device, but they are kind of expensive and they want to charge you oversized shipping too.
It makes me wonder how many people really know how tall their horse is, and how many just guess.
Cloudkisser, my colt is 6 months old now. I'm going to try the off-side of the weight tape to get a guess at his current height. The weight tape says he weighs 565 right now.