Splatter - I was using Red Dot. Can't seem to find my notes, but I recall I started at 13 grains and worked down to somewhere around 10.5 or 11 before it was consistently sub-sonic. The one BIG thing that I do recall is that at 13 grains of Red Dot ("The Load", as it's known), I thought that was too much pressure. The primers fairly flat, and I had some brass flow into the case-necks. If I was doing it again, I'd start more like 12 grains and work down.
Though, actually, I wouldn't do it again, and don't really recommend it. It's probably dangerous, and it turns out that sub-sonic 30-cal's (800-ish FPS my Chrono said) without a suppressor are still too loud to shoot without hearing protection (IMHO), and I can't think of any other reason I'd want to load a sub-sonic round.