There is a fixed relation between the clock and the tach but that can differ from one tach to another (by construction).
It can usually be read somewhere on the tach, but not on the scale, have a look on the backside of it.
It should say something like "avg cruise 2292 rpm". That means that if you run the engine at 2292 rpm for one clock hour then the tach will show 1,0.
If you run it faster the tach will show more for one clock hour. Slower than 2292 rpm will show tacho less than 1,0 for one clock hour.
Overhaul on engine and other components is by airborne time, that is what you put in the airplane logbook, clock time between takeoff and landing.
Happy flying
