Just wanted to say thanks again to Joe for that formula. Had some time to spare while I was at school the other day and decided to lug my 8x10 wooden pinhole camera out for a shot. I bike to school so it was a hassle carrying the camera to where I went and riding at the same time, but having the formula in hand gave the assurance I wasn't going out and making a guess. Here is the final product:
I metered for the sky and then added an extra stop of time because I tried using a yellow filter for the first time.