There is a formula used to get the correct amount, I used to have it written down, it uses their actual age and weight to work out how much fluid they need rather than going on just age.
80-100ml per 500g of body weight - eg: A 3.5kg baby - 3500/500= 7, 100ml x 7 = 700, 6 feeds per day 700/6= 116ml per feed.
HTH