I'm 6'5" so I understand completely the situation you're in. That being said, my backpack ALWAYS goes under the seat in front of me if I have 2 bags and one is already in the overhead. I only make 2 exceptions to this rule. 1) Once everybody has boarded and if there is still space in the overhead above my seat, then I will put my backpack up. 2) If I'm flying in BE and there isn't any under-seat storage big enough for my backpack (i.e. 777 or 757), then I will put it in the overhead simply because I know that there will be enough space for everybody's stuff. Other than those two exceptions though, I am of a similar opinion to the previous poster who said that everybody gets one spot in the overhead.