I think flight length does a lot matter and 1-stop should be enough, more stop more hurt on LF.
I acquire Brittannia early to fly SIN-LHR route. It takes 23hr outbound and 21 inbound by B175. The LF is around 30-40% with Y36/C20/F2 config.
Later in game, when I've got B707 (Y120/C10/F2). Thing has changed. The better flight length will get more LF on the same route. But I also think that even you fly Jet with the flight length of 22+ hrs. It won't get much customer anyway. Until the newer technology is suited the route, you might plan it then.
BTW, I still run my SIN-LHR route on Brittannia. They still make money around 23,XXX/round trip. When I see my profit is at my satisfaction. Then I reconfigure my plane to suit what customer needs to Y80/C3. LF is rise back around 70% with the same profit.