K,Derate is simply using 'spare' runway to save the engines..
If your takeoff weight is anywhere near your limit forget it..If your 'actual weight'is light use the runway.Go into your charts at the actual weight check the temp alongside to see the 'saving' difference from Actual temp.In view of the improved 'braking'these days on the big boeings,one might not have to 'change the Vspeeds.Maybe the Analysis chart 'shows' the different(from actual,)speeds..As I said before,the 'up'ed V1 and V2 were to put the aircraft closer to the higher temp's position in the ACC/Stop runway case..
