Just suggesting to use a mic instead of a caliper. I make my living using calipers and mics--I'm a machinist. A caliper will get you reasonably close but a mic is infinitely more precise. I wasn't suggesting an inside mic but simply measuring the outside diameter of said seat post. If I have a tolerance of .005" or more, a caliper is fine. Any thing less than .005" requires a micrometer. It's too easy to get a false reading with a caliper. Using either is an acquired feel--especially the caliper. If you push too hard or not hard enough your reading can be off by as much as .002-.003" which in precision machining is enough to drive a freight train thru.
Jon