Liquid intake with exercise

I am consuming the normal 64 oz of water daily, but I have added in 2 hours of cardio exercise a day, so I sweat a lot and drink about 2-4 cups for each hour I work out.

Is there some kind of equation of how much extra liquids you should add in? For example, for every hour of intense work out, you should add 16 extra ounces? 

I'm having trouble deciding if I am still getting in enough liquids.

I am by no means 'selling' this tool but I have used it in the past to figure hydration out 

http://nutrition.about.com/library/blwatercalculator.htm

it's interesting for sure to see how much you need based on different criteria than just the standard, 8 glasses a day