Put the subwoofer AT the listening position, play bass heavy tunes. Start crawling around the room, and listen for the spots where it sounds best. These will be your candidates for sub position. Do this when no one else is home so that you are not committed to the loony bin by loved ones.
I also agree with going with lowest extension. I believe that higher tunings allows for greater output, but since you don't need so much output with a modest room, you get to have a bit more extension . . .