Back in the stone age we used what was referred to as an "Air Mike" for the really precise measurements, and centerless grinders for the finish sizing. The air mike could read to a millionth inch, and since it read the flow restriction on a column of air it automatically blew all the dust away, and the oil film as well. Now that we have laser interferometers we can measure to a billionth of an inch.
Haven't messed with this since the 70's and not a machinist expert like you and Dave, but I recall an optical measuring device, while not sure it read in millionth's, I am pretty sure it would get down to 1/100,000.