I just never know how to measure for a seatpost - the clamp area at the mouth of the seat tube is meant to be slightly oversized in order to accept the post, and measuring that diameter and rounding down to the next common size seems crude.
I once searched the earth for the proper post for my Hardrock circa '96, thought it was 31.8 or something - wound up 30.2 per the shop. I've never heard of a 30.2, but they had one.