Hello again,
Rather unconventional of me maybe, in partly answering my own question. I understand there are two main methods of achieving thrust reduction on the Boeing 757-200. The first, ATM or Assumed Temperature Method, only applies exclusively to Takeoff Thrust. The standard rating is 1.72 EPR. So a max reduction of 25% gives an EPR of 1.29.  Please correct me if I am wrong?
The second method is a Fixed-Derate, which is applicable for climb thrust only. The available Standard Climb Thrust is 1.42 EPR . On the Thrust Mode Setting Panel (TMSP) pushing button 1 on the ground arms the FMS for a 6% thrust reduction in climb 1 mode usually occurs at 1000ft AFE default, typically a Ndapt2 departure which is 1.39 EPR. If you push button 2 on the ground, you will arm the FMS for a Climb 2 thrust reduction of 12%, which results in an EPR of 1.36. This value will show on the upper Eicas display in white (armed) not actual (Green). You can only influence the climb settings on the Takeoff page of the CDU by altering the deceleration height, Ndapt1 3000ft AFE, or Ndapt2 1000ft AFE. The thing is as far as I know you cannot enter a Takeoff Thrust Rating that is lower than the lowest climb rating (Fixed –Derate 2). This explains why when raising the OAT temperature on the CDU Takeoff page and reducing the EPR Thrust setting when reaching a value of 52°C is max, the CDU will not accept anything higher, because the resulting EPR is the same as the Fixed-Derate2 EPR of Climb 2 or 1.36 and will not go below that value. Hmmmm……any wiser?
Swedsim

In fact, ATM is only available for the takeoff, while fixed derate can be chosen for both takeoff and climb. In the former, the engine power is reduced to the value at which one may start at maximum with the planned TOW, but keeps the reserve up to the maximum possible thrust value. For a Fixed Derate it is assumed that you have a "smaller engine version". The maximum thrust value is then really limited to this lower value without reserves, while in the case of the ATM method, the thrust levers can be moved completely forward and can use the available reserve. It can even be combined for the takeoff both methods, but depending on the airline handled differently or allowed. Similarly, the use of the ATM is usually not allowed if the Rwy is contaminated or wet.