This past weekend I was adjusting the "action" on my bass guitar, which means adjusting the height of a guitar string within a few thousandth of an inch. The guide I used recommends doing this with a precise scale and a magnifying glass so you can read the marks. To me this felt rather imprecise. When adjusting the neck tension, the gap was small enough (i.e. under 1mm) to use automotive feeler gauges. Why don't they make something similar at larger thicknesses? The answer is, they do: guitar picks. I had a random assortment of picks in an Altoids tin nearby. The thickness of each nicely marked on the front. With a little futzing around and some calipers, I found a stack of picks to match whatever gap I needed.
Afterwards, to save myself some trouble in the future, I generated a table of measurements to pick stacks. The color codes match Everly "Star Grip" pics and Dunlop Tortex Std. picks. The 'mils' column is U.S. thousands of an inch.