The best way to determine the proper exposure for any pinhole film and camera combination is to test. It will use up a roll of film; but the time and money initially expended will pay off in a future of good exposures.

The exposure calculator that is built into my Zero 6x9 gives a good indication of proper exposure, and provides a good starting point for the test. It is easiest to do the test on a bright sunny day with a subject that includes a full range of tones (both lights with detail and darks with detail). Make five exposures ranging from two stops below to two stops above what the calculator indicates. When the film is processed, evaluate visually on a light table to see which exposure best captures the full range of tones in the subject. This is the proper exposure on a bright sunny day (the EV 15 situation noted above). This exposure time is a "magic number" for your camera and film.

As mentioned above, in darker lighting situations, subtract the EV of the ambient light (EV 15 above) to determine how many extra stops of exposure to give. Let's say your magic number is 5 seconds and the ambient EV is 12, you'd need to make a 40 second exposure.
5" = magic number
10" = one extra stop
20" = two stops extra
40" = three stops extra

Doing the test to determine the proper exposure on the pinhole camera itself eliminates the need to factor in the film's reciprocity failure characteristics since the test exposure was so long. In practice, I've found this approach to work great up to six or seven extra stops of exposure.

The testing can continue by doing prints or scans to determine the best film exposure; but doing the visual examination suffices.