Yup, its a photocell that counts the interuptions, not the duration of the interuption. Since the middle of the prop center of the prop and the tip of the prop are turning at the same rate you can tach it anywhere, its not measureing the speed which we know will be greater at the tip because its got a further distance to travel in each revolution.

You may not get a good interuption of the light at the tip but its going the same RPM as the rest of the prop