If you want easy, and don't want to mess with feeler gauges, then here is something to remember.

35 mm film is 5 mils thick. (0.005")
120 film or 220 film is 2 mils thick.
4x5 or any sheet film is 7 mils thick.

So, I actually use pieces of film for quick calibration. The error introduced by the coating on the suppor is so tiny it is not worth mentioning.