Hey! I'm sure you must have figured it out by now, and if so, let me know what conclusion you came to. I asked myself a similar question last fall and then noticed what could be an element of the answer.
I think one way to do this is to position audio in regards to the frame itself.
I can't remember what movie it was exactly, but I noticed that a ringing phone was positioned behind the character and this character was facing away from the camera (with the phone being at the foreground). My intuition would have been to place the ringing sound in the back speakers (because the phone is behind the person who ears it), but I could only hear it in the front speakers.
I think the reasoning could be: since the phone was in front of the viewer (me), the sound should also be in the front speakers.
For your example, following what I've described, I would put the sfx in the front speakers if the sound is generated by something on the ground (since your camera is pointing towards the ground).
Then if we were to hear an airplane, for example, without seeing it, I would assume that it is higher than the camera, and therefore behind it. So then I would place the airplane sound in the back speakers.
My ultimate instinct though would be that this is not only a technical manipulation but also artistical. maybe my observation was good for one movie, but another one would have mixed the audio differently. Similar to a director using a subjective shot (using the camera like it is the eyes of the character), when another director would use a shot showing the character instead.
Does that make sense?
