Gah! Okay, take it to a bike shop and get them to size your seat tube w/ the tool designed for that job. It's a cylinder that incrementally increases in diameter by tenths of a milimeter. You really don't want to trial and error a seat post.
this all got me thinking where the seatpost i was trying came from.. and i realized its from a frame thats 27.. so false alarm.. maybe i will take you up on that post.. but even an ugly ass cannondale might be too pretty for that thing