Originally Posted by
Andycapp
Digital Caliper-
Dial Caliper-
Vernier Caliper -
Any of the three of these should be more than accurate enough in my experience. If you know how to read the vernier you probably have the skill needed. As far as actual post diameter being under by .1mm or so, it has to be (that's only .004"). A "size to size" fit would surely result in a stuck post.
Seat posts are typically undersized a bit (plus there is roundness and cylindricity to consider). Cheap posts are usually undersized more than high-end ones (for obvious reasons). I have found Campagnolo posts to be the most true-to-size and consistent of all brands of vintage posts. A 27.2 Campagnolo post will typically be about 27.18 or so (when new, and used/worn, sanded-down, and beat up doesn't count, sorry), for example.