What I am saying is that the Boeing limit is an EGT limit, not a thrust limit. Airlines then convert this limit into an SOP. The airline I work for its the same as most other airlines. TOGA thrust for 5 mins. But that has not come from Boeing, its come from the airlines. The EGT limit for MCT is 790 according to that chapter. I said you COULD argue (not that I WOULD) that you were not at the limit, so a time limit would not apply, but we are now into the realms of legalese.
Back to the OPs question. TOGA is for 5 mins, derate isn't. If you now want to know if the derate thrust setting is above MCT you would have to select CON on the TMSP and see what its giving you, or look it up in FCOM1. You would then have to make a decision. Do we not think this is slightly unfeasible during a V1 cut, maybe in the middle of an emergency turn or dealing with a terrain issue? It seems, having spoken to other 757 operators that Boeing thinks so, and that the SOP in most airlines is that if you select TOGA you have a time limit, if you remain at the derate setting then you do not. If your training department says otherwise that you will have to go with what they say. But I would probably question their priorities.