Derated Take Off and Assumed Temperature 

A Derated Take Off may only be made  with good weather conditions and no windshear reported. Its purpose is to save wear and tear on the engines. It is worked out in manuals by engineers allowing for the runway gradient and length, + weight of the aircraft .  In addition the height of the airfield above sea level. An assumed temperature is then computed. Add on flight simulation programs allow you to use Derated Take Off. You do this by adding the Assumed Temperature into the D-To box on the FMC. Capt Mike Ray advises that you cannot work this out in flight simulation and should use the standard temperature at 15 C. The problem is you may not generate enough thrust to get you over the trees at the departing end of the runway. I have added a few degrees to the the current temperature and this has worked quite well. At least now you are aware of this subject. Without the manuals and airfield data you can see for flight sim purposes this is not going to be easy, at least you now know what this is.