GearDown&Locked,
Essentially Flex/Assumed thrust is calculated by examining the temperature at which, for a particular weight and set of environmental conditions, the aircraft would be at it's Maximum Takeoff Weight, and then operating the engine to the lower Pressure / EGT / RPM limits associated with the higher environmental temperature.
To address the twice repeated remark - "assured performance no worse than the certified hot day value", actually performance is BETTER than if operating at the actual higher temperature. Whilst the engines are producing the same thrust, no credit is taken for the lower TAS associated with the same V1/Vr/V2 on the cooler day. For example, on a 20°C day with an assumed temperature of 50°C at Sea Level, and a V1 of 140 KIAS, TAS at V1 is actually 141.2 Kt Vs 148.3 Kt at the higher temperature. Kinetic energy to be dissipated in a rejected Takeoff from V1 is thus only 90.7% of that if the actual temperature was 50°C.
Regards,
Old Smokey