My informant works part-time for a small-sized consulting firm, and takes a lot of readings and data measurements as part of his job. He hears this metaphor frequently when being assigned to do these readings, especially when he wants the data to be as accurate as possible. He also uses this metaphor when critiquing the work of interns. For the success of the consulting firm, it is important that data is read as accurately and precisely as possible. My informant explained that if two consulting firms are competing for a contract, and one company’s readings are taken in tenth of units, and the second company’s readings are taken in hundredths of units, the second company will likely get the contract because of their attention to accuracy.
Although he’s heard and used the metaphor many times, my informant cannot remember where he first heard it. He interprets the metaphor to be used as an indication of something of very small size, and that this logical reasoning is likely what has popularized this metaphor. If a gnat is small and an eyelash is small, then a gnat’s eyelash must be very tiny. He also knows he has heard the phrase used in two ways: 1) with ‘within’ to indicate a small margin of error, and 2) with ‘as small as’ to describe how miniature something appears.
I have also heard this metaphor with respect to taking and recording data, and I believe it’s commonly used as a clever way of saying something commonplace in dull mathematical fields.