Given how amazingly hard it is for some people to find their proper stock shoe size, with some seemingly needing many, many iterations of variation on size and width before getting it close, I can't imagine playing around with this on custom or MTM money. On simple Allen Edmonds, I've speculated on going up a width or down a half size or whatever, tried this idea in a shop and realized that I was way off base. 1/16" error in a shoe is alot.
I once ordered motorcycle boots, Bates Fastlanes, that needed slight customization to get over my fat calf. What they sent was about two sizes too big and they were completely uncooperative about what I saw as clearly their fuckup. I've done an AE webgem, because I know my size there on that last, but this is MTO.
I know with suits, that a tailor is not just using a tape measure, but there is all kinds of more complex understanding of shape, motion, what's flattering, customer style, the exact materials used, the construction and all that. Cut and sew by number is in no way the same.