My thoughts: go for a few psi below the max listed on the sidewall and chalk the tires. Reduce from there based on how far you are rolling over the tires. Obviously the front tires will roll a lot more. So you'll probably end up in the upper 40's up front, and tune the rear to get the handling balance where you want it.
Good luck, you're the first person I've seen post up on autoxing the new focus, so let us know how you do