The seat post fits the internal diameter of the upper portion of the seat tube, while the clamp for the front derrailuer (sp?) fits the external diameter of the lower portion. Since the walls tubes can vary in thickness and some seat tubes taper as they approach the top tube, it is not a good idea to use seat post diameter as a gauge for the size of your clamp.
A set of calipers should be used to measure the seat tube where you want to install the clamp. I would take the bike to a local shop and have them measure the tube and have their best mechanic make a suggestion as to which clamp would be best. If you purchase the equipment at the shop, there should be no charge for this service.