Oh, so it isn't rigid? That wasn't obvious!
Gauges aren't usually made of plastic!
As I said above above a cheap telescopic bore gauge will give you repeatable accuracy of 0.02mm if you use it just below the seatclamp slot.
It would avoid this:
The ring gauge put it at 27.4 mm but if the wrong size has been in there for so long its safe to assume the top is bent in slightly and the 27.6mm would be the better size even though it might take a little persuasion to make it fit.
Which is what I meant by this:
I mean it measures it at the top of the seattube, where it's most likely to not be round!
But yes, if the cycle mechanic know the limitations of that 'ring gauge' and what to check before using it, then it's probably simpler to use.