I'm having more success with marked headers when:
I press the "shoot button" when the ball is already about 3-4meters away from my player, and I release "shoot button" when ball is near my player's head(1-2m away). Not too early, not too late.
According to Hitman´s impressions (an mine) it seems to be absolutely irrevelant if you double press it or not in certain moments, it´s just important you do it damn early. Looking forward to some testing runs as soon i come back to my PS4.
I declare the "PES2019 Manual Header Research and Testing Laboratories" opened.