I have u last in suede, calf, and shell. I had to size down by 0.5 in suede. I wear the same size in both calf and shell. Calf felt a tad tighter at first, compared to shell, but over time (1 yr wear) they stretched more than the shell. It depends on the shape of the feet, etc of course, but for me sizing down 0.5 for suede has worked. They're all oxfords (not derbys) by the way. Hope that helps.