With most operators I've flown for their local XAA allows derates & assumed temp. I then flew under another EU XAA that did not allow this, but allowed only the assumed temp reduction based on full thrust. I thought this very simple. I then wondered why bother with the derate and assumed temp at all. It requires more paper charts for every derate, or more data in the I-pads. It has the possibility of errors in choosing the wrong derate table.
Does anyone have definitive performance answer why using derates & assumed temp is better than just assumed temp from full thrust? I know there are operators with different max thrust, e.g. 22K, 24K, 26K, 27K. It would not matter as max thrust would be the datum. Are there some operators who have mixed max-thrust fleets. If so, would it not be simpler if using ass-temp from max thrust?