Hi novicef,
Think of a Derated engine + airframe as a different model to the same airframe with with a non derated engine.
see
http://www.skybrary.aero/bookshelf/books/2263.pdf 7.2 Derated Takeoff:
“AMJ 25-13 / AC 25-13
(4)(b) Derated takeoff thrust, for an aeroplane, is a takeoff thrust less than the
maximum takeoff thrust, for which exists in the AFM a set of separate and
independent takeoff limitations and performance data that complies with all
requirements of Part 25.”
In this case, “the thrust for takeoff is considered as a normal takeoff operating
limit.”
For a derated takeoff, the limitations, procedures and performance data must be included in the Aircraft Flight Manual (AFM). For each derate level, a specific RTOW chart can be established for a given runway, taking into account such new limitations as the minimum control speeds.
We used to use Flex on top of the derated figures when conditions allowed (i.e. no contamination).
If you are operating from a contaminated runway and are using derated figures or full rated figures then you can't flex.