thinking some more about the other channels, it's possible that if the nearby AP is on say channel 36+ and transmitting heavily, that the AM may get into a bit of trouble hearing low rssi frames on 44-48 due to the adjacent channel interference coming out of the nearby AP. So with that in mind, the 3ft/1m is probably a reasonable place to start, and increase the distance away from active APs if you have a low number of AMs (which makes sense anyways since you want the AMs to be equidistant from the APs and around the coverage area).