Morning Chaps
Here's one line pilot who's not surprised or shocked at Mutt's post.Our company and many others in the UK use Max derate all the time
unless there's a sensible reason for not doing so.(eg Field limited at Funchal perhaps
)
From memory Handling the Big Jets reckons there's one percent margin for every three degrees the assumed temperature is higher than the actual temperature.(Looked it up just to check).
Many times in the 757/767 the allowable reduced thrust temperature is less than you can wind into the derate anyway.Less powerful a/c are closer to the real limit more often.
Lots of factors to consider and at the end of the day its airmanship again.