I know this is going to sound dumb, but here goes …

I have measured the inside diameter of the seat tube with vernier callipers. It’s coming out as 27.0mm, so I need a 27.0mm post – correct?

It’s an old 90s GT frame and 27.0mm seems an odd size to me. I have 26.8mm in my head as being the correct size. It has recently had a shimmed USE post in there. The shim is all scuffed so no size measurements on it. The shim was showing as 27.0mm on the callipers too, but I’m thinking that the shim will uncompress a little when removed from the seat tube, so that measurement may be unreliable.

Should I get a 26.8 or a 27.0 seat post? I’m thinking the smaller of the two.