The page with the data that my earlier post refers to has been taken down. But here is a post about Ektachrome 100 films in the era of 100G, and some numbers for calculating reciprocity that should get you a lot closer. The numbers (Schwarzschild exponents) were derived from standard astrophotography tests that test for the amount of reciprocity failure at 128 seconds of exposure time, and are typically good within about 1/3 stop. 100G was one of the Ektachromes with the least reciprocity failure, about equivalent to the best of the Fuji E6 films regarding reciprocity failure.