I shall share my ingenious method I used this week, if only for amusement...
Rather than try to measure the diameter to within 0.2mm without the proper callipers, especially given the tapered end to the post, I opted to measure the circumference. As circumference is Pi*diameter each 0.2mm step is a 0.62mm step in circumference which is much easier to measure to the nearest likely value. Wrapping the post in a piece of paper (as an actual tape measure would be too thick) and then measuring that a few times until I had a fairly confident figure seems to work fine. It only gets sillier as when I held the old seatpost next to the new under the bright kitchen halogens, I noticed it actually had the diameter measurement stamped on it, which I hadn't been able to find when I looked in the garage, but at least it was correct
Check for a stamp on the post if you haven't already and maybe check if the bikes still listed on the web.