I recently bought a frame on craigslist that came with a seatpost already installed. Unfortunately, despite being 0.2mm too large, the seatpost had been rammed in there by the previous owner. I've sanded down a stem before so I'm game for that, but it's something like 350mm long...I'd like to cut it down so I only have to sand a smaller portion. In determining the new minimum insertion length could I simply look at other aluminium seatposts I have and use their min. insertion length as a proxy?

Also, if I decide I don't want to do this, what sort of shop would I go to to shave off a couple millimeters from a post?

Thanks.