Private_Flyer
Percentage of power is used as a way to describe desired performance
The rule of thumb is use
- 75% power for max cruise speed
- 65% power for "normal" cruise
- 55% power for economy or to maximize range.
For simple fixed pitch prop trainer/touring aircraft Cessna POH's give cruise performance in tables so you simple select the cruise altitude and temperature, decide on what percentage of power you want to use and you will get the RPM you must set and the resulting cruise true airspeed and fuel consumption.
Piper unfortunately insists on using charts so you first have to find cruise speed and fuel flow for the specified conditions and percentage of power than go to another chart to figure what RPM setting this corresponds to.
(Note later models did have a handy table which gives you RPM values directly, on the windscreen sunshade but I would not count on this as it was rather flimsy so a lot of club aircraft no longer have an original (or even any sunshade))
A few other points.
1) Percentage of power does have application to some engine limits. For example both Continental and Lycoming do not recommend leaning below 75%of maximum rated power.
2) The POH cruise speed and fuel consumption figures were mesured on new perfectly rigged aircraft. There is no way a 10,000 club beater is going to make book figures. I tell my students to reduce the book cruise figures by 10% and add 10% to the fuel flow figures when nav planning, and
3) fuel flow figures are for engines running properly leaned. If the engine is left full rich it will burn up to 25% more fuel than the POH figures
Frankly I am rather surprised this was not covered in your PPL training.....