@pinhole_dreamer: Go to http://www.mrpinhole.com/calcpinh.php and click on "pinhole exposure guide"... of course, you still need to calculate recoprocity for your particular film. If you get a good exposure with guesstimating, I see no reason for complicated calculations, though. I usually just note the Sunny 16 exposure time from the pinhole calculator and guess from there: A little cloudy is 1 stop more, shadow is 2 stops, dark clouds is 3 stops, and so on. When the exposure is several seconds to minutes, a second more or less doesn't really matter anyway.