Learn to hear when the engine sounds smooth - the tach is just good for recording -NOT setting
anyway - to read props -- use white fingernail polish and paint last inch of backside of prop white -
avoid really direct sunlight - but stand behind prop and move tach back and forth and at angles to the prop till you find the sweet spot - it really may be back a foot - reading will be very constant and repeatable.
The least expensive little tachs such as the Hanger 9 -read as accurately as the most expensive ones you can find.
The reason is the same as why a 10 dollar watch is as good as a 10,000 dollar one -for accurate (accurate enough) time
Current electronics have made this type of instrumentation dirt cheap and accurate.