This performance is all very simple really as long as you input the correct data. This nonsensical discussion of flex (used by EK)/derate (not used by EK) and TOGA shows that many here don't have any grasp of 'Performance A' and associated requirements.. And yes, a heavy 345 taking off with flex will often be rotating in the reds after a 4000m take-off roll, but as long as it clears the 'screen height' by 35' after an engine fails at V1 that's all that matters!!
As for the video on youtube a few posts back - if you listen they're just seeing 'how it goes' at TOGA when the aircraft is light.. Trust me, it's off like a scalded cat at light weight and TOGA...