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.
That is, (22K x 75%) = 16.5 == 36.5% reduction from 26K.