I see no weakness in your purchase. Have fun with it. There is always a little reciprocity failure with all film at such long exposures, but don't worry, a little overexposure of Type 55 never hurts. I don't use any formulas - just trial and error. At most I've ever had to extend an exposure is about a minute for a 5 minute exposure. In the 10 secs to a minute exposure range, the RF is in the seconds in my experience.
I use a "Black Cat" exposure guide. Most meters don't have an f/stop range of a pinhole camera. I take an exposure reading, say 1 sec at f/64. I set the guide at that reading. Then I go to the aperture of the pinhole, say f/256, and look up the new exposure time - 30 sec. Easy as that.
All you need to do it look up the f/stop number of your pinhole body cap. My Nikon pinhole body cap is f/185.
Hope this is helpful. My web site shows a lot of pinhole photography (www.walterpcalahan.com) if you go to the experimental button on click on PinholeDC.
Originally Posted by Magpie