Also, FWIW, the term is "derated" thrust, which is a fixed amount of thrust reduction used for takeoff to improve engine life. Depending on specific airplane, engine, and airline, fixed derates may be used instead of variable "reduced" thrust.
For fixed derates, check the book to see if you can take off with the desired derate (usually 2 available), push a button, and the engine or autothrottle computer automatically calculates and sets the selected (nominally 5-20%) derate. For variable reduced thrust takeoffs, the engine setting (N1 or EPR) has to be calculated for each takeoff.