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)

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.