To a degree the position I use depends on the fuselage shape. I’ve seen people put overflow fittings just in front of the air intake and they wonder why there is fuel everywhere each landing. Most of mine end up 90 degrees to the flight path, but generally to one side. So on your round airliner fuselage between 4 & 5 or 7 & 8 O’clock so it’s not lower that the bottom of the fuselage in the event of a gear up landing and also so I can see/access it. I always switch on/prep my models from the Left, so the fitting is below that side. If you can angle forward that’s great, definitely not angled back. If the vent line reaches the top of the tank in a loop it won’t siphon on the ground.