Yes, sounds like you know what you are doing then.
Guess the only tips I have are to make sure you have them stand on a flat, level surface, and measure from the "hoof" up to the last hairs on the mane.
I also make sure to measure mine right after they have been trimmed by the farrier.
Are you using a measuring stick with a level arm, or just trying to eyeball it?
It is surprising how much difference you can get in your height measurement, so I also measure the same horse a few times and take the average....
But if you are not worried about them being registered I would not make a big deal of it.
If you are though, and you think they are close, it's worth the bit of effort.