For the 9 pm show, I'd recommend at least 45 minutes to an hour (the secret has gotten out and I have found that I'm not finding this spot available as often). For the 10:30, it should be easier once the place clears out from the first show.
Right. For 10:30, I'll bet you won't even need the trick. Just get there at 9:15 or so, and wait for the crowds to leave after the 9:00 show. You should get a good seat.