Two decades as a "professional" pilot, and the last 5 in an aircraft that has flex (A320), and embarrassed to admit that I have never heard/thought/seen of this subject...
I always assumed(!) it would take the actual temperature for the speed and distance calculation, keep reducing engine power till ASDR/TODR hit ASDA/TODA and calculate an assumed temp to give me that required power, will definitely read up on the way flex is calculated on the bus.