How much water you need depends totally on your sweat rate, which varies a LOT from person to person. The best way to know if you're drinking as much as you're sweating is to weigh yourself before and after your ride. "A pint's a pound, the world around" - every pound in the difference in your weights is a pint of water that you need(ed) to replace. Of course that doesn't address how well hydrated you were before you weighed yourself the first time - but it's at least a way to know what you lost during your ride. It's good that you're keeping track of how much you're drinking, too - that will help you figure it out.

Re: sodium - actually, sodium helps you retain water. When I get hyponatremic (which I do very easily if I'm not careful) I pee like crazy. It's my body's way of trying to increase the concentration of sodium in my blood ... but of course, the result is that I get dehydrated on top of hyponatremic. Any more, I usually figure it out pretty quickly... but I still get caught out once in a while.