The correct fit is a snug running fit BEFORE tightening the clamp bolt. If you insert the post and can rock it, it's too loose.
Another indicator is the cut slot. On a perfectly sized post it'll barely narrow when the clamp bolt is tight. If it narrows by over 1mm, it's too sloppy.
Good fit is important beyond simply keeping the post from sliding or twisting. Since it's only secured at the clamp, an undersized post will be free to rock as you shift your weight, causing added stresses.
BTW - seatposts are sized to the tubing ID as shipped from the mill. Reaming serves 2 purposes: correcting for undesized tubes, and/or correcting for ovalizing during welding or brazing. An overly large ID usually means that tube was too oval, and now has thin walls in places.
Last edited by FBinNY; 11-15-22 at 08:28 PM.