Thanks for the quick reply.
Is there an easy way to know which cities are gateway cities? I assume ORD works for LHR because i can get a normal ticket which routes BOS>ORD>LHR.

Since the stopover in ORD is more im portant thatn the stopover in LHR (or the open-jaw), I understand that I could do: BOS>ORD stopover ORD>LHR>SOF, then SOF>LHR.

thanks again