Originally Posted by Andrew R Stewart

I don't like to judge a tube's OD by its ID. Especially in bike making where seat tube reaming is a common step. The comment about the post size having a .5mm range suggests a poor ability to measure. The best way to determine a post size is with trying various sizes of posts. Since the top of the seat tube gets a lot of heat and often in two or more separate procedures the ID can, and often does, deform so a reaming can be needed to get the ID back to round enough to fit and hold tight a post. Sometimes a bit more reaming is needed and the post size is picked to fit that. It is not uncommon to have the ID at the top of the seat tube to be smaller than deeper in the ST, or better said the post that fits the top of the ST might not be the best fit further down the ST.

By a 30mm head tube I assume you mean the ID is that. This is a spec that the headset's press fit calls out. Again the after reaming ID has little to do with the OD. Remember for the same OD a bike tube can have different wall thicknesses. Andy