I typically wear 9D and 9.5D in most American sizes.
So something with a narrow last (5-last, Allen Edmonds), I usually take a 9.5D, but for something a bit more wider than that, I take a 9D.
Not sure if this helps.
What a strange thing everyone is saying you should size down in Clarks DB! I wear 8 in Jack Purcells, Superga, Red Wing, Walk-Over, etc. comfortably with socks, and the DBs in size 8 I just got from Amazon were so small I could hardly slide my feet into them. And when I finally managed to the tongue was stretched and creased like crazy. My arch may be higher than average but if the Clarks DBs do run large why are they still so small?
^ The label says Men/Hommes 8M (US), 7F (UK), 41M (EU) so I guess they are what they are supposed to be.
I have also found that the heel is very narrow. The length, however, is perfectly fine. In any case, with such narrow heel and width I would think they run small rather than large as the consensus says.