Yup, you're right about the diagnostics. I don't remember all the figures for 2,3 or 4 blades but I recall my instruction book says hold the tach up to a fluorescent light. I've not tried it with an incandescent light (bulb).

The test doesn't actually prove much. Just that the circuit and photo sensor is working. Being digital, there's not much to go wrong, either it works or it doesn't. I doubt if a standard digital hobby tach can go out of adjustment.

Holding the tach about 2" in front of a light satisfies both conditions for the tach to work. Plenty of light and a stable pulse.

In real conditions ie taching a prop, there are many variables. There's the available light and the contrast offered by the prop. EG taching a gray APC prop in cloudy or dark conditions won't give enough contrast to allow the sensor to see the prop.

For the battery lamp/torch/flashlight method to work you need to hold the lamp on the opposite side of the prop directly facing the tach.

These tachs really work best in decent sunlight were the tach is facing the light, not in shadow....

The good news is, in Mexico there should be plenty of bright sunlight