This explains the principle


The exact settings depend on a few things for example what kind of blades you use.
If you use modern blades like the ones from align then a slightly higher head rpm makes the heli more precise to fly at the expense of shorter flights.
The stock wooden blades are "half symmetrical" and those give more lift at lower head rpm but those are less suitable for inverted flight.
With those blades and some careful tuning you can reach flight times up to 30 min on a tank.
If budget is less of a issue then you could use a "governor" like the futaba gv-1
Those keep the head speed constant for you and you use a channel to tell it what rpm you want the head to turn at and it will control the engine throttle for you.
One way or another you will end up spending time on the field making test fligths, change the throttle curve, test again until it behaves in a way you like.

enjoy