MOSFET thermal derating
Usually the mosfet data-sheet will have Power De-rating curve, like this one 


Which i got from the AON6403 MOSFET Datasheet, I would recommend you check the datasheet of your device for a similar curve and utilize that and pay close attention to the condition under which the curve was derived. 

Otherwise you can do an approximation, de-rating means that you modified the max parameters of your device to fit your operating limits, for thermal derating the operating parameter will be the junction maximum temperature (Tjmax) (Note, that Tjmax might not be the appropriate value, since reliability will go down the closer you run to junction maximum temperature, maybe someone can provide a better guideline for that or maybe there is some safety factor that you need to apply) Now we now that a 1st order approximation for Tjmax is 

Tjmax = Tambient + Power*Rja. 

To derate solve re-arrange for 

Power = (Tjmax - Tambient_max)/Rja. 

This is just an approximation because Rja is completely dependent on your foot print layout, PCB, heat sinking, air flow, etc. a good place to start is the datasheet listed Rja value when mounted in the 1 square-in FR4 substrate, and then make assumptions that will hold valid for your design.