even my best digital calipers with multiple readings only gets me in the ballpark. I have a graduated sizer, but it's a jeweler's gauge intended for ring sizing, so doesn't go big enough for 27.2 and beyond...and you're right that it won't account for a deformed or ovalized opening. The T-gauges combined with an accurate caliper is really the way to get down into the tube and get an accurate measurement. My set is nothing fancy but they work for me.
Hijacking: What do folks use (preferably a common hand tool) to make a deformed seat tube opening round again?