Visit this site as well: https://stellafane.org/tm/atm/test/tester-2.html
where you can download a pdf file which is called “atm_ronchi_screen.pdf”, to be printed with a laser printer, without magnification (that is to scale 1:1), obtaining a lattice on transparent square with side 2 inch (51,8mm) composed of 100 lines per inch, which corresponds to 3,9 per millimeter.