Print at 100% / "Actual size" (no "fit to page"). Then check the calibration bar at the bottom measures exactly 100 mm with a ruler. To use: lay a stamp edge along a row of dots and slide until the holes line up with the dots — the number on the left is the gauge (holes per 2 cm). Measure all four sides; different gauges across vs. down means a compound perforation.
Free printable gauge from StampID — identifystamp.com. Identify any stamp from a photo (country, era, catalogue listing & value) with the StampID app for iOS & Android; the catalogue match usually tells you the expected perforation.