You should drink plenty on the day of the flush even though you stop eating at around 2.00 pm you can continue to drink but not within 30 minutes of taking any of the Epsom Salts or potion. Remember that the Epsom Salts isn't taken just to clear the bowel it also relaxes the bile duct and stimulates the pancreas among other things. One hour after the last Epsom Salts has been consumed you can start to drink re-hydration fluids but water on its own won't re-establish an electrolyte balance. The easiest way is to dissolve a little Sea Salt in the water, drink fruit juice or consume a commercial re-hydration drink. Being dehydrated means that the electrolyte/water content is inadequate and generally speaking you can't increase the amount of water in the body without increasing the quantity of electrolyte as the ratio tends to remain constant.
Don't be disheartened by the flush it was very successful by the way you describe it. Stone counting is a distraction from the real business of getting the body clean.