i'm not so sure about dry fasting - i haven't tried it and i'm not sure that i ever would, since i have enough problems with weakness and fatigue when i water fast. what i would say is this though: drink according to thirst. on my last 14-day fast (which was about 9-days ago now) i was much more thirsty than i had been on previous fasts. some days, i found myself drinking more than a litre of water, in small sips throughout the day. on other fasts, i sometimes barely drank a glass of water in a day.
i believe that your own body is your best guide. if you're thirsty, drink - and if you're not, don't ;)