I'm using a Cane Creek Crosstop lever; the Cane Creek web site says it's 26.0. It's never slipped on my bars at all, and I don't need to use a shim or anything. Maybe it's working because I have a right-hand lever mounted upside-down on the left-hand side.
I haven't seen my Vernier calipers since I moved in June, so I can't help you there.