Step 1: Feed runway info/conditions and meteo data into the computer
Step 2: Computer looks for the highest assumed temperature from either F10 or F20. In other words, lower thrust. whichever gives the highest assumed temp wins.
Step 3: In case of both F10 and F20 giving the same assumed temp, then the computer looks at which gives the highest RTOW (regulated takeoff weight)
Step 4: In case both F10 and F20 give the same RTOW, then F20 wins.
Note: If the computer is not used, using published tables always uses F20.