It always depends on the last for me. For the 348 I am a full size down from Alden's Hampton last. On the brannock I'm a true 8.5. I wear the same in Alden's Hampton, but 7.5 on the C&J 348. Milage may vary with different lasts.
I had tried on a 48 Last at Leffot about two years ago and actually had the size written down, but I never actually tried the size I picked. Tried a 7.5 narrow and an 8 narrow and went with a 7.5 regular width, which Leffot didn't have in stock.