Well, you asked Vince, so here goes.
This is MY reasoning BTW so I am not saying others are wrong. Shaft diameter is highly subjective.
I personally like 11.75 mm. Why ? B/C the shaft still keeps its rigidity yet allows a good reference for aiming. (Dang, I said it,but that is really why), 11.75MM comes out to be right around .460 inches. This allows a .060 sight line with approx .200 on each side,(left & right) of the center .060 strip. I don't know if others see this but I do. That .060 center strip gives me shaft center and also happens to be an approximate area of contact between the CB & OB. This allows me to pinpoint aim, adjust for spin, and measure amount of English.
When I ordered my OB shaft, the rep told me that maple shafts tend to lose their rigidity,(stiffness) around 11mm. Meucci users years ago would turn their shafts down claiming to get much better spin at the expense of a "whippy" shaft,(no longer rigid).
PERSONALLY, I like 11.75 with a super-pro taper.