I'm certainly not an experienced flusher but from my understanding is that you shouldnt need to limit you're water intake on a flush day; in fact, it appears that many people drink water until 1 hour before the oo/grapefruit juice concoction, since many experience the dehydrating effects of the Epsom Salts . You just want to make sure youre stomach is clear of water when you take the oo mixture so it doesnt sit on top of it.
Lemon juice in water is beneficial but not required.