about on average, how much runway is used for such a takeoff, (are you into the reds and whites, generally?)
Derate/flex is calculated specifically for every takeoff, using TORA to calculate the minimum take-off thrust required. With derates of up to 73 deg on the 777, and similar flex on the bus, almost every longhaul takeoff (and indeed a lot of midhaul takeoffs) end up in the 'reds and whites'.