A good place to start testing, but I haven't yet done so myself with these films. If you can find the right conditions, or can create them with ND filters and f-stop changes, you should be able to know if you're close in very short order. Just make a base exposure at a level that reciprocity doesn't come into play, then (with the same subject if possible) make exposures at a few longer times and then compare resulting negative densities.
Originally Posted by Perry Way
You've probably also seen Gainer's method with a fixed exponent of 1.62, which should also serve just as well as mine, but has the option/advantage of making a sort of 'slide rule' chart for field use.