The most reliable indicator of hydration is the color and quantity of urine. It should be light colored, and there should be enough to make you feel like it’s time to pee at least a few times per day, preferably a couple more than that (especially for your experiment). Coffee will throw this off because it’s a bit of a diuretic, inducing urine production for reasons other than excess fluid. I drink coffee in the morning, but only after about 12-16 ounces of water. Boy oh boy do I pee!
J-pouchers only have one universal disability: reduced water absorption. If you don’t take care of that problem you aren’t likely to do as well as you could.