@ jiejie:

And before I forget and leave this thread, Many Thanks once more for your excellent advice !
As it turned out - and as you already assumed would happen - our UA flight departed 50 min late from SFO (the boarding process was chaotic and the gate area like a zoo ) and consequently arrived more than an hour later as scheduled @ PEK.
Luckily I choose your advice and had booked the connecting flight early the next morning.