Often there is a water decal on factory original tachs, within which the reference RPM is given. It could be different for different tachs, as the tachs are manufactured for different engines, with each engine's specific characteristics in mind.
A look at the tach back case behind the panel might yield this information for you.
The other way to find out would be to stopwatch the tach while you're flying the plane. To the right of the ot the tenths digit, you should be able to see the hundreth's "wheel". It may be half black and half white. Experiment with RPM, and find out which RPM causes this wheel to turn at that rate.
Pilot DAR