I'm a "true" - I use that word lightly - 12.5B on the Brannock (I can wear a 12B on Barrie/Trubalance). I ordered these in a 12C, which is one width up from my Barrie size. I thought maybe the extra width would give me enough room for a good fit, but the Leydon is a relatively narrow last so it's still too tight. I can try a 12D, but that might end up being too wide without giving me the length I really need.
Ideally I would like to try a 12.5C, but I don't think I can get that in an exchange from NRO. A 13 would almost definitely be too long, so looks like I might be screwed because I can't get a refund from NRO, only an exchange. If I can at least get a 12D from them to try on but they still don't fit, they will certainly end up on B&S here!
A 12C and a 12.5B should be pretty close. Wear them around the house on carpet or with shoe covers. See how your feet feel. NRO should be able to get a 12.5C. I will say that while the 11C was tight on me at first, the 11D was loose and sloppy from the start.