I've got a project bike, that began as a cheap Micargi Triplet Tandem. I'm looking to upgrade the front seat post to a 350mm post, from it's stock 300mm post. I put my digital calipers on the existing post, and it is 28.50mm diameter.
I've found places that I can buy 28.4mm posts, and 28.6mm posts, but 28.5mm doesn't seem to exist. My guess is that I should buy a 28.6 post, and expect that it would fit based on my "farm logic" that all other female parts measure just under their advertised size, so get the next bigger size. Plus, in a "worst case" scenario, I could probably shave .05mm off the post in a lathe to make it fit, assuming that should hurt the structural integrity much.
But, from experience, I've learned it is a good idea to post here to see if there is a better solution that my guesswork has found before I start getting out the heavy equipment in the metal shop.