probably the most appropriate tool for that is a telescoping hole gauge and a micrometer or a caliper. I bought a Chinese set thinking that Starrett can't be all that much better, but I ended up getting a Starrett later. Too many sources of error to use a butt checker. One other problem with a butt checker is that you might not be able to trust the tube to be the right size. I noticed a Columbus SFL tube was tight in the seat lug, but didn't check the id. Never had a bad one. Turns out that bike now has a 27.4 seat post in it because the tube wasn't drawn properly. Live and learn, should have measured the ID before I brazed it in.
Using a telescoping hole gauge isn't foolproof either. Takes a light touch and seat tubes aren't usually round.
Last edited by unterhausen; 06-29-20 at 10:44 AM.