http://tinyurl.com/gahxv

This is too much for me to try to paraphrase- basically you weigh yourself, ride for x amount of hours, keep track of how much you drank, weigh yourself when done. How much weight you lost (the longer ride, the better) x 15.2 is your fluid deficit in ounces. Then you subtract your deficit from your total fluid consumption, and then divide by how many hours you rode, and you get your rate of fluid loss per hour. Once you know that, and you know sweat and urine have about 50 meq of sodium per liter, or about 1000 mg per quart, you can replace what you are losing!