Draw a graph, starting at sea level and 15 deg C, this is the International Standard Atmosphere (Or part of it..), Boeing allows you to takeoff at temperatures 39.4 degrees hotter than that temperature.
Now if you draw a line at 1000 feet pressure altitude, the ISA temperature will have dropped by 1.98 degrees, once again mark that point plus 39.4 degrees.
Keep going until you get to 8,000 feet, reducing by 1.98 degrees every time you increase by 1000 feet.
You have just drawn your environmental envelope. YOU CANNOT takeoff if the OUTSIDE Air Temperature is outside this envelope.
However you may fool the engine into thinking that it is hotter for the purpose of takeoff calculations.
Its different per aircraft, but lets use ISA+45C for your reduced thrust. Once again mark this point on your graph at 1,000 feet. You will see that its 5.5 degrees hotter than the envelope temperature.
To make sure that you dont take off at these temperatures, they are marked with the letter A.....
Mutt