Help! I’m hoping to start showing soon, and I’m going to need a pair of tall boots. I have a 13.25" calf which is only 16" high and a 8.5 foot, with very very thin ankles. I can’t find tall boots!
I tried ariats and the ankle was beyond baggy! I’ve heard tredsteps (I’d get the Donatello) have a nice tapered ankle and thin calves, but the height for my foot size and calf size is almost 3" too tall. I’ve also heard mountain horse has tapered ankles (some better than Tredstep, some say) and their height looks good for me. Do you know of any other brands with thin ankles and a shorter height? Thank you!
I have skinny calves and ankles (but am tall), the DeNiro Auroras fit my skinny calf and ankle great and you can buy in different heights (I have X tall), so you can probably get them shorter. SmartPak sells them and you can return them no hassle if they don’t fit. 
https://www.smartpakequine.com/deniro-ottaviano-aurora-field-boot-8847p
Parlantis
Don’t forget to leave some room for the boots to drop. that will help you out with the height
Sergio Grasso might be worth a try, although they’re pricier than the other boots you mentioned. Dover sells them but doesn’t show the full size chart… here it is (height/calf in cm)
https://www.colliervillesaddlery.com/newsg.JPG
I have the exact same problem but can’t afford customs unfortunately. I walked into Dover and said, “These are my legs. Find me boots.” I tried on a pair of Middleburgs and they fit like a glove. Leather is very soft, shine up pretty well, and are surprisingly comfortable. They are the closest to a custom look for my tiny ankles and short calves ever. The best part? $200. I will definitely be re-purchasing them.
Which Ariats did you try? I seem to remember that the cheapest model (I think it was the Heritage ones?) were quite loose in the ankle for me, but the old Challenge IIs had thicker leather and fit almost perfectly. The tack store I went to sold thicker insoles which I got just to get my ankle up a bit higher into the thinner part of the boot. Took a bit of tweaking, but it was way cheaper than getting customs.
My calves are 13 inches around and I need 16 inches tall and skinny ankles. I have mostly had customs but got a very good fit with Sergio Grasso and they are a pretty boot too.
I would definitely say parlantis. I have a 12.25" calf and a 16" tall leg and they fit me perfectly.
