How do pilots calculate the temperature to put Into the FMS for an FLX takeoff? I recently read an article about derated takeoffs and I understand the concept, but how do pilots know what temperature to put into their FMS if they want an FLX takeoff? Some people say it's a rough estimate like ~50-60 C, but is this true? If it is gauged by some external factor, what is it?
For Airbus Aircraft you use a tool from Airbus where you put in data like wind, air temperature, pressure, weight, runway characteristics (length, heading, elevation, etc)  and other elements that calculates the flex temperature. It also gives you some other parameter like required runway length among others.