FL390
The problem with your premise is that right now neither American or Delta fly the 747 nor have any plans to in the future, and it looks like United and NW arent rushing to get new orders in. It seems like the 777 is the plane of choice which is what Boeings long range forecasts predicted. So then, one may ask, why is Boeing embarking on this 800 seater? it may be a huge gamble but in one fell swoop it would make the a-380 much less competitive now and much more so in the future! interesting times these are!