Mark, the most basic point would be to try to have the M3s separated by about the same distance as your distance from them; then toe them in slightly so that the tweeters point at your ears. Another point is to position them, if possible, so that that distances from the M3 mid-woofer center to the floor and each of the walls are different from each other, so that the boundary effects don't pile up at one frequency.
That calculator is rather simplified and appears to just deduct 6dB for each doubling of distance, and that actually isn't the whole story, since room effects have to be considered. You should have plenty of power, so just listen and enjoy the music.
