Gear shift.

Out of pure curiosity, which would you say is the best time to shift gear in order to achieve most power for longer? I have a manual, 1.8 GTI. I’ve heard some people say shift just as you hit the red (as seen in the picture) like the DSG box and I’ve had others tell me to shift just into the red.. anyone able to give me a decent explanation as to what’s best?

https://uploads.tapatalk-cdn.com/201...74938f5e58.jpg

Where ever rpm peak power is - only disclosed by a dyno or engine data .