A standard credit card has a thickness of 0.75mm, so it's tough to get exactly 0.5mm. A standard sheet of 20lb paper is very nearly 0.1mm thick, by comparison.
For industrial purposes, you should be fine making sure that the strands extend far enough past the ferrule barrel to verify that the crimp is good, but no so far that they are obviously too long (say, about 1/32" or so). I usually strip my wires exactly to length, but use a little extra push on the wire while crimping in order to make sure the strands extend far enough.