Some other pinhole sites are:
http://www.mrpinhole.com/index.php
http://www.pinholeresource.com/shop/home
http://www.pinholephotography.com.au/Theory/theory.html
http://www.withoutlenses.com/

You can use the formulas, but they vary, as others have said. I just find that f200 to f300 work well. Also, as SMBooth said, thinness is good.

Keep working at it, pinhole is fun and you're off to a good start. Next April, do something for Worldwide Pinhole Day http://www.pinholeday.org/