As I understand it, the formula calls for 1/2 your body weight (in pounds) converted to ounces. So a 150 pound person would drink 75 ounces of water.
Assuming you weigh 9 stone:
9 stones = 126 pounds
(one stone = 14 pounds)
126 pounds = 113 ounces of water daily
(33.8 fluid ounces in a liter)
113 ounces = 3.34 liters of water
I don't know how our bodies are designed for this, but I've been drinking at least as much water as this figures out to be for several years, and it's never-ever been anything but beneficial :)