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.

Do you mean how much water should you give baby on top of formula milk? If that's the case, I reckon there is no real need to give baby additional water because there is already water in the formula you made. Too much water in a baby's system can cause serious health issues, such a kidney failure and problems associating with the brain.

As Idza mentioned, you should always follow the ratio of water to powder according to the back of the tin. I do give my baby a little water (about 10 to 20ml) because I find that his formula milk is a little too heaty.

thanks