A perforation gauge is not a measuring tool. It can be used to help count how many complete perforations fit in a 2 centimeter interval. It would be necessary to use a Kiusalas Specialty Gauge or the more modern Multigauge based on the distance between perforation holes to the nearest 1/000 of an inch. A standard metric perforation gauge cannot distinguish between flat plate perf 11, hole spacing o0.072", and rotary perf 11, hole spacing 0.070", or rotary perf 10 spaced 0.080" or flat plate perf 10 spaced 0.079".
Try comparing it against a few other different perforation gauges since some gauges are not as accurate as other gauges. Also, some used stamps have a tendency to get bent out of shape after they are pressed hard to dry after being soaked off.