Half your body weight? Are you sure? If you are 240 pounds then half of your body weight would be 120 pounds of water which is over 14 gallons of water! Are you sure it wasn't an oz for every two pounds of body weight (which would be 120 oz of water in your case)? I believe that this is a recommendation made by either Atkins or the Eades in their book.

The other common rule of thumb is 64 oz plus an addition 8 oz for every 25 pounds of weight you want ot lose.