<blockquote class=quoteheader>Matt Corker wrote (
see)</blockquote><blockquote class=quote>
I have a digital vernier (callibrated!) - I've used it to check lines a few times and I must say it's usually close (not closed).</blockquote>
I'd imagine it would be close, calipers are fine for general measurement but not very good when looking accuracy to less than...... say 0.002" (0.050mm). They also tend to have a knife edge near to the tip, which could squash the line with very little pressure and lead to an inaccurate (undersize) reading.
Adelicate 'feel' with a decent micrometer should give a more realistic value for line diameter.