I apologize, chysun that i haven't been able to get back to you... hope everything is going well with you and your colt.
Here, in the western U.S., the method I described to you has, generally worked out well... my experience has been that it's accurate to within a half a hand or so. The string should be held against the lower leg... "follow it" for the most accurate measurement. It works well on any colt. I've tried it on yearlings up to about 14 months old and it still approximates the "finished" size.
I have to say, here on the ranch using working cow ponies, they tend to be a bit smaller... near 14 to 14 1/2 hands... maybe 15 in stallion colts. Our horses are thick in the hips and short coupled... all from the Appaloosa/Arab blood that we favor for sturdiness as well as super-smart (for a horse). I suspect that working with taller breeds, especially the Thorobreds can throw the estimates off... and just as I explained before and reiterated by Ice.Maiden, a well fed, well maintained horse will do better in growth than a neglected one... I can tell by your posts that your foal won't be neglected.
Again, sorry about tardiness... Keep on truckin'!