Print this page at 100% scale — disable "Fit to page" / "Scale to fit" in your print dialog. The square outlined in red must measure exactly 50mm per side once printed; check it with a ruler before use.
Place the printed marker flat on the same surface as the crack, close enough to appear in the same photo, then use Upload (not the in-page Camera capture) so the photo is analyzed with real millimeter measurements.
Red-outlined square = 50mm per side