The most difficult part of understanding baker’s percentages is understanding that 100% does not refer to the total weigh of your recipe. Rather, it represents the total weight of flour in your recipe. If your recipe calls for 300 grams of all purpose flour, then those 300 grams represent 100% of the flour. If your recipes calls for 50 grams of whole wheat flour and 250 grams of all purpose flour, then the combined weight (again 300 grams) represents 100% of the flour.
When a recipe is stated in baker ‘s percentages, everything is stated relative to the flour, which, as I have said, is 100%, or 1. Thus, if the baker’s percentage calls for 75% water, then you simply multiply the weight of your flour by .75. If your recipe uses 300 grams of flour, then 75% water is 300 X .75, which equals 225 grams of water.
The reason bakers use baker’s percentages is that it makes it easy to adjust their recipes to meet their needs. Although bakers make may bake dozens of loaves at a time, the same principle will work in your kitchen. Say, for example, you discover you have 437 grams of flour and you decide you want to use it all to make bread. If your recipe calls for 75% water, then it’s easy to calculate that you need 437 X .75, or about 328 grams of water.
To see this in action, take a look at the Basic Sourdough Recipe that I have posted on this site. I show it in baker's percentages, but I also show how these percentages convert to actual weight. I have it listed for one loaf and for three loaves. A few minutes study will show how the math works.