Seatpost binder bolt
 

Probably a simple answer, but...

When trying to order a seat post binder bolt for my Bianchi Ibex, I was given a number of length options, but when I took my calipers to the frame I was not sure where to measure. I measured the outside of the flange but that was not one of the options on the web site. If I have a 26.2 mm seat post any ideas on what length binder bolt I need?