
With a light meter and a calculator you can determine the difference in stops required by the aperture number of the pinhole.
Suppose you’re using ASA 100 film and the meter reads 1/125 second at f/11. We suppose that you’re using an f/158 pinhole.
The formula is
Difference in stops = 2*ln(f1/f2)/ln2
The constant factor 2/ln2 = 2.885, so we can simplify the formula to
Difference in Stops = 2.885*ln(f1/f2)
Where f1 is the aperture number in the meter reading and f2 is that of the pinhole.
Then in the example
Difference = 2.885*ln(11/158) = 7.7 stops.
The light is reduced (hence the negative sign) by 7.7 stops.
So, disregarding reciprocity failure, you should increase the time by 8 stops. Thus your new time, uncorrected for reciprocity failure, is 2 seconds in this example.
Of course you should increase the exposure even more to correct for reciprocity failure.
If you’re using TMax 100, the 2second reciprocity correction is approximately 1/3 stop.
See the table on page 5 here:
http://www.kodak.com/global/en/profe...4016/f4016.pdf
You can consult the film maker's data for the approximate reciprocity correction required for the film you use.
As others have commented, the amount of time to add to compensate for reciprocity can only be determined acurately by testing with each exposure time used referenced to the meter readings.