Normally there's not an exact spec, merely that a mid-range reading is OK.
Production variances (engine, wiring, the gauge itself), oil viscosity, RPM, and temperature also affect it.
If you want to assign a # to each hash mark, you'll need a mech gauge for comparison.

Years ago, 911 gauges had readout #s, and some owners would annoy the Service guys if their gauge differed from what they thought they should be reading.
Porsche removed the #s, and the official line was "if it's in the OK range, it's OK, now run along".