Is there a ratio to follow for the number of prepared formula milk vs how much water I should give my baby for a day? (For future reference)

You should follow strictly on the instructions given on the formula's milk label. Actually you do not need to dilute the formula milk by adding a little bit more water, as nowadays formula milk are manufactured not so concentrated as compared to the past. This is what my PD told me, not to dilute any formula milk or give baby below 1year old to drink water.