Fuel tank balance

Piston engines use percent power. So if you want to cruise at 65% power
(and burn fuel at the rate stated for that power setting), you look in
the aircraft manual for the rpm setting and manifold setting that gives
65% power (there may be more than one). For any given percent power,
you will get better fuel economy if you use the lower rpm and higher
manifold pressure setting. (This is akin to going up a hill in a car in
a high gear and full throttle. Such procedure uses less fuel than
downshifting and using say, 3/4 throttle).

There is no percent power guage that gives a direct measure of the
engines output.

You can also derive percent power from fuel burn if you have an
accurate fuel flow meter. The rule of thumb is 12 horsepower per gallon
per hour. So if you are burning 12 gallons an hour that is 120
horsepower. If the engine's max horsepower is 180 then you are at
120/180 percent power.

Things like this give pilots on long flights something to do....

Thrust is something a little different. Airplanes with jet engines use
thrust (whose unit is pounds) for their power settings, I believe. I'm
no expert though. I do know you don't use thrust for small gasoline
driven prop engines like in Cessnas and Pipers.