When doing Baker's Math you specify each ingredient as a percentage of the flour weight. Hydration is defined as the percentage of liquid in a dough based on the flour weight.
This calculator will tell you how much water to use in your recipe if you know the flour weight and the starter weight and you want to acheive a particular bread hydration. You can calculated this in either grams or ounces. The calculator assumes the starter is always a 1:1 ratio of flour and water (this is called a poolish). Hydration is expressed as a decimal value from 0 to 1 where 1 means 100% hydration.
This calculator will tell you the flour and water weight if you know the total weight of your ingredients and the desired hydration.
This calculator will tell you the total weight and the hydration of your recipe when given the flour, water, and starter weights.