Ted, although you studied this carefully, it isn't clear how the answer came out M3s, since your room is bigger than average and you're going to be sitting an unusually long distance back. As chess commented, the M50s might have seemed a likely candidate if you were looking for the slightly more recessed M3/M50 sound rather than the M2/M22/M60/M80 presentation.
Nevertheless, if you do proceed with the M3s, the location just a foot from the corners should be significantly better, as Mo explained, since you'd have close to the generally suggested equilateral triangle arrangement with the mains being separated about as much as your listening distance from them. Mounted that high it ordinarily might be suggested that they be angled slightly downward so as to have the tweeters pointing at your ears, but that may be unnecessary because of the sitting distance, especially if they're positioned upside-down. If they're on shelves, have them forward enough so that they're at least even with the front of the shelf, to minimize reflections and refractions.
