James, it's not that complex....ie, you don't have to calculate activity indices, or whatever, because the kidneys are in there controlling everything 24/7. You really don't have to worry about controlling your water input, other than having common sense. It's a long, long physiologic story, but the bottom line is that the kidney will preserve your body water status and also make you thirsty when you're getting close to the limits of its ability to do so.
There is no set volume that's right for everyone, but an average urine output is around 1.5L, so (insensible losses--sweating, respiratory, stool, etc.) 2.5L is probably an average water input. Keep in mind that when carbs are burned, water is formed in the process, and that most food contains significant amounts of water.
Practically, as far as the need for water goes, it's just flat sort of evolved into what is done today--a camelbak isn't worn on a cool day of trail riding for 3 miles, but is for 23 mile desert loops. All just common sense--the 'kidney' will do the rest.