One problem with simple formulas seems to be that the effectiveness of washing FB papers (not so much film) seems to decrease markedly after an initial period where most of the hypo is removed. The initial period follows a simple exponential rule very well, but then the washing slows because of the way thiosulfate is absorbed into the paper fibers. I would expect RC papers to behave more like film. The curves given in the above reference show that thiosulfate is essentially never fully removed from FB papers, although acceptable levels are achieved in an hour.