Sadly, the Ecote Corset boot from Urban Outfitters didn't deliver. Super budget-friendly and made of a soft faux suede, it unfortunately doesn't tighten nearly enough to work for a thin calf. Pulling those laces as tight as I could, I was only able to shrink the circumference down to 15 inches, which is smack in the middle of average range. Even with the laces tied, I could slip the boot on and off with ease (even fully tightened, the ankle was a roomy 12") -- not a great sign when you're looking for a custom fit. I'd go so far as to say that with laces loosened just a bit, this actually makes a nice wide calf boot.
A better bet? Ages ago a very thin calf reader of my boot pages tipped me off to the Gee Wawa Blair, a quite narrow back lacing style that is almost impossible to find, but you might check ebay for this cute boot at a fraction of its original retail cost. Unfortunately, back lacing boots tend generally more toward over-the-knee pirate-y or high-heeled "boudoir" than riding. But I'll keep you posted if I find any others that get it right!
Calf slimness: */5