Pratice, practice, practice standing him up often, have others approach him and touch him around the shoulders. Get ahold off an actual measuring stick with the base and level and practice placing it and swinging the arm over and lowering it teaching Pony to hold still and relax. A tense, fresh Pony is a bigger Pony then a warmed up, relaxed one and when 1/4” is a make or break measurement? You need to properly prepare. Pick a venue for the appointment Pony is familiar with and try for a warmer month. Doesn’t matter when their feet are worked on and shoes can be pulled but theres a vet there and it must jog sound, it is subject to drug testing as any show entry is.
You know, i got a measuring app that just showed up in my phone with an update, the tile is just called “measure” not official of course. But just opening it and scanning gives you a pretty good measurement and the more you understand about things that can make a Pony “ grow” or “shrink” 1/4”, the better.
It removes any excuse for not knowing your horses approximate height too, which is a great help in estimating weight and tack sizing as well. I can sure tell you it has been a huge help when looking at sale horses and we aren’t talking 1/4” off the sale ad either. Pretty useful.