Typically you want the seat tube ID to be about .2mm larger than the post size. You can ream to various sizes, even removing all the .8mm butt. Whatever you decide just be sure to check for available seat posts. There aren't many 30.0mm or 30.6mm seat posts around other than a Thomson and some elchepo's, but anything from 30.0 to 30.6 should be viable.