Lowering the pitch and tightening the ridge line before putting any tension on the sides seemed to help a lot (see results below). I was hit by two heavy thunderstorms that evening -- not a lot of wind, but a TON of water. Unfortunately, I pitched the tarp on a nice flat spot at the _bottom_ of an incline. Also, since the pitch was a lot lower, the serenity shelter was a little bit saggy in front. In the end, I got a little wet, but not because of the tarp pitch.
Just for fun, here's the serenity shelter standing alone when the weather was much nicer. That was good sleepin'.