I'm not implying that there's better data around now, only that people who read this need to confirm or adjust through personal testing. If there is better data, I haven't seen it. And I'm disappointed that Reeves took down his color and B&W film information.
Originally Posted by celetron
Bond's methods were exhaustive. Reeves uses a rather short and simple test that I use. It's outlined in his and Michael Covington's books on film Astrophotography. (Be aware that the Covington book 2nd ed. has a typo that I found last year in one equation used to calculate reciprocity, so check his website for the errata page.) This method uses a 1/8 second exposure, then 128 second exposures under the same conditions but with a 10 stop neutral density filter and some bracketing. The amount of bracketing in stops that produces the same density at 128 seconds (with filter) as the 1/8 second exposure (without filter) is used to calculate a Schwarzschild exponent. This is a check at a single data point, but it's very good at giving a useful indication of the relative degree of reciprocity failure in a film. You could also use other ND filters of different values to get more data points, but 128 seconds is a good start.
This testing takes only one reference frame and then however many bracketing shots you feel you need, at whatever step rate in stops would be useful to you. Four frames and reasonably careful work will give you a very good start.