I really don't think you can calculate dehydration like that. I started my last 7 day dry fast at 152.8 lbs. In 1 week I lost 21 pounds, which was roughly 14% of my body weight. However, all I had was a slightly dry mouth on day 7, and I am not a super human either.
Ultimately it all depends on how much liquid body holds as well as whether someone is a water drinker or not. Personally I pretty much only drink water to break fast and normally don't consume more then maybe a 1 to 1.5 liters of fluids on a daily basis.