What is baker's percentage?
In baker's percentage every ingredient is written as a percentage of the total flour weight, and the flour is always 100%. A dough with 1000 g flour, 700 g water and 20 g salt is 100% flour, 70% water and 2% salt. The total is more than 100%, and that is how it should be.
It makes recipes easy to scale and compare. Once you know a baguette is about 68% water and 2% salt, you can make any amount of dough.
Hydration
Hydration is the water percentage. 60–65% gives a firm dough for bagels and sandwich bread, 68–75% a typical rustic loaf, and 75–85% the open, holey crumb of ciabatta and many sourdoughs.
Sourdough starter
A 100% hydration starter is half flour and half water. For the most exact hydration, add half of the starter weight to the flour and half to the water when you calculate the dough.