I've found the Zero Image calculator to be pretty reliable, but anything over a second I double it, and anything over 10 seconds or so the calculator that came with my 4 by 5 Zero Image says to multiply it by 10 ... yikes! With ASA 100 my standard daylight exposure is 2 seconds, with kentucky windage for shadows, color of subject, my mood and so on. They all come out.
With the 4 by 5 I'm discovering that light fall off at the 25 mm setting is so severe I need to double it again so the edges don't go completely thin on me. I also have a Zero Image 2000 and their 135 (cute little bugger!) where this is not so much of a problem.
But as everyone else here said, experiment. When in doubt, add a few seconds -- I've found that pinhole seems much more forgiving of over exposure than a regular camera, probably because of the reciprocity stuff.