Hi, I just started my Disney - diet, but here is me 2c.
Before I started my diet I asked my doctor this very question. He told me that the 8 x 8oz. glass was just a guess. Some people need more than 64oz and some way less. You also absorb water from foods with high water content such as fruits and dairy. You do absorb water from all drinks such as juice and coffee it is just the sugar and caffeine doesn't let you absorb as much as straight water.
You will have to ignore the ickyness but this is what he told me.... Take a look at your pee before you flush. If it is light yellow you have had enough to drink. Really really light you don't need that much water. And dark yellow drink more water right away. If you take a lot of vitamins it will throw the color off, especially C.