Go look at NW's timetable. PEK-NRT is 9:05a 1:50p
From NRT to the US, they depart ~3:00pm to 4:00pm, and arrive in the US from 8:35am (PDX) to 2:40p (late DTW).
How much earlier do you want? Do you want a PEK flight to depart at 6am (like its flight from BKK) so that you have to get to the airport at 4am? And how much earlier do you want your flight to arrive in the West Coast? Do you really need to get back to the US at 6am?