Quoth checkers:
There is a the consideration that, with at least the FAA, the maximum flex/thrust reduction you can have from full-rated is 25% (thrust, not N1).
So, if you compute your takeoff performance with a 22K full-rated takeoff and then apply a maximum 25% reduction, then you can have a deeper effective thrust reduction than if you used a 26K rating.
Very true.
However - the greatest economic benefit (in parts life) comes from the first few percent (say, 10% thrust, or 4% N1 IIRC). Merely going for the greatest reduction - the highest FLEX temp in other words - may turn out to be false economy because of longer TO times, and higher total fuel burn. Not sure if anyone's really pursued in-depth analysis of this issue; a good subject for someone's thesis perchance.
