I know jet-powered airplanes very rarely use 100% available power for takeoff, and most often reduce power to increase engine life. How does a pilot set thrust to an exact level?
The airplanes I'm referring to are common commercial jets such as A320/B737 types.
If they'd set it manually with thrust levers, there are some complications: there are no placards on them. If you try setting revs via the displays, revs and temperatures do not change instantaneously, and looking at the indicators and adjusting the lever would be a dangerous distraction during take-off (imagine: the engines roaring, the brakes are applied and tires are rubbing against the asphalt; or the plane is rolling, and the pilot is looking at the gauges).
On the other hand, if a limit is set via FMS, how does it know when to add more power if it's urgently needed?