I also use the Starrett expanding ball gages, although it takes a while to develop the proper feel for them -- just like the telescoping gages. It helps to drill/ream a few test holes you know the diameter of, and measure them for practice. The feel you get in a particular hole depends in part on the surface finish, too, which adds another variable into the equation.
DR's right -- the most accurate way is with a set of pin gages. Even with those though, if the hole surface is rough...what "diameter" are you measuring?
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