How much water is needed for flour dough when making chapati
When making chapattis, the recipe might give a quantity of water, but it will probably also say you might not need it all. So you should add it gradually. 

For example, the recipe I use says for 225g (8Oz) of flour you use 110ml of water "very approximately"  and add it gradually "until all the flour is incorporated and you have a soft, pliable dough". Then the dough rests for 15 minutes. I actually have my doubts about the conversions in this recipe, but the method is more important so it always works. 

Quotes are from The curry secret -- Kris Dhillon