Existor, I would order your normal size and a pair half sized up. Then decide which fits or not. I tried the Simpson Cordovan Boot and it fits perfectly in the same size I am wearing Forest (fits excellent), Uetam (some heel slippage), Llubi. However, it was Cordovan. Suede felt more snug, so probably gonna have to size up.
What I am really confused about is the lacing, I once tried the Chukkas on Detroit and the lacing was too tight/snug. So I figured the last must have a too high instep, yet the Oxford/Derby (don't remember) fit pretty good and the lacing was almost too much of the "V".
At the same time the Chukkas on Rain were gine yet the lacing on the Rain Oxford way too snug. Pretty confusing!