On 27 February 2013 15:55, James Paige <Bob@xxxxxxxxxxxxxxxxxxx
<mailto:Bob@xxxxxxxxxxxxxxxxxxx>> wrote:
Oh, one more thought! This question got me curious, and after some
googling, I saw a nifty suggestion on stackoverflow. In addition to
displaying a ruler, your calibration screen could also display photos of
some common coins, since whoever is doing the calibration might have an
easier time locating a coin than locating a ruler.
Obviously you'll get a more accurate calibration with a larger object,
but for some purposes, I guess speed beats accuracy.
Another option would be to use a standard paper size, e.g. 'make this
line match the short edge of a piece of A4 paper'.