When I build (converted, actually) mine, I used the pinhole calculator already posted and drilled the hole in brass shim stock by hand with a tiny drill bit (#79, 0.0145") and a pin vice. For exposure times, I've had pretty good luck with two Android apps: LightMeter for the initial EV, and Pinhole to calculate the exposure for the pinhole camera. The latter app allows you to save the details of your camera so you can simply call it up and consult the exposure tables when you need them. Both are free and available through Google Play (no personal interest, just a satisfied user).