I agree with Nick. Getting the pack wet is minorly important, even here in the north eastern part of the country (where humidty is usually higher than 60%, 'cept in the winter.) I am far more concerned with keeping stuff in it dry. Water resistance, top closure, etc would tell me more, I think. You might have missed some of the data that would make it more usefull, IMHO. Not just weight picked up with wetting, how quicly it dries, but also, how well the water penetrated, and what the age of the fabric was.
Silnylon does well for water resistance, drying time, and weight pick up, but fails with abraision and puncture resistance. Good when new, but not so good when it gets a little older. This suggests an age test to me. Difficult to test reliably or with any degree of repeatability.
Your test seems geard to a single type of fabric. A single non-laminated fabric, such as a traditional pac cloth, was never designed to be very water resistant. Water entered through out the cloth, and, the seams. Dyneema was waterproof only because many (most?) were coated with PU. In conjuction with other cloths, some were fairly water resistant, some picked up a lot of water weight, and, some allowed penetration into the pack body. The newer laminates, cuben/nylon hybrids, may indeed pick up a lot, but retain full waterproofness. A 100% cuben pack will also not allow much, if any measurable, penetration into tha fabric. Drying time would be pretty irrelavent to a cuben pack. Indeed, I am not sure if wetting is actually correct, since it will pick up some weight from rain, but, never actually wets the pack, not to start a symantecs sub-thread.
So, they are independent variables. Something not captured well by simply weighing packs before and after soaking them for some standard time period. My opinion only, but water resistance becomes as important as the water retention and drying time of the material. And all three numbers would be influanced by the number of "typical use days" of the fabric. A simple wet/dry weight doesn't really capture the info that I would want in a review.
Water retention and drying time can be computed as you have demonstrated.
How to compute water resistance? This can be very subjective... Perhaps asking for a general opinion on how to feasably test for this might yield some results. I would suggest simply dunking the pack in a vat of water for some time period (not including the top, of course) and weighing it against a fully wet pack? Of course, this means repeating the measuments, again.
Perhaps something equally simple for an "ageing" test. Say 5 hours drying time in a commercial dryer with a half dozen new 1-3/4" childs blocks would simulate a seasons use? Framed packs/stay stiffened packs/internal framed packs would need to have been removed from the frames, stays and internal frames removed, of course. Generally a lot of time to perform this sort of test, repeating the whole set of measurements, again.