A bit on lithium and fluid levels in the bady from Drugs.com...

"Maintain adequate fluid intake by drinking 8 to 12 glasses of water or other fluid every day while taking lithium. Vigorous exercise, prolonged exposure to heat or sun, excessive sweating, fever, diarrhea, or vomiting may cause dehydration and side effects from lithium. Call your doctor if you lose a significant amount of body fluid as a result of sweating, diarrhea, or vomiting."

So, the advice above "1/2 your body weight in ounces of water" should work great!

DJ