Small point, I know, but....
Engine speed and prop speed are, in almost all piston-engined aircraft, exactly the same. So it doesn't really matter which one you relate your tacho time to.
In those engines where they're not the same (e.g. Rotax engines) the prop is driven by a fixed gearbox - double the engine speed, and you double both the prop speed and the tacho speed. In the case of your 150,000 prop rpm/hr tach, and my 2.2 ratio gearbox, the tach could be described as a (150,000*2.2=)330,000 engine rpm/hr tach and it would mean exactly the same thing. In fact, it would make more sense, since my RPM guage measures engine RPM, not prop RPM.
As for MP, no, that makes no difference. So if you pay by the tacho hour, you'll be best flying at high MP and low RPM. Which is interesting, since that's not exactly an economical way to fly in terms of engine maintenance costs!
Hope that (at least kind of) makes sense!
FFF
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