Playing this game backwards, if you are planning a takeoff on a relatively cool day, at a lesser TOGW, you can ask "What is the hottest ambient I can stand and still be legal at this TOGW?"
Once this temperature is known, you can set thrust equivalent to what the engines could produce on the hot day (i.e. reduced thrust), and have assured performance no worse than the certified hot day value. (Remember, the wing is still flying in cooler air)
So how is this Assumed Temp calculated?
GD&L