Perhaps I have the wrong end of the bull, so to speak. I always thought "flat rating" refered to the maximum OAT at which the engine would produce rated power.
For example the ALF-502 produces 6970 lb thrust at sea level at up to 15 degrees C, that is, it's Flat Rated to 15 degrees C. Where as the LF507 produces 7000 lb at sea level to 23.3 degrees C, that is, it's Flat Rated to 23.3 degrees C.