I think the biggest obstacle to understanding baker’s percentages is the tendency to think of the baker’s percent as the percent of total. This is not the case. The percent is the percent of flour. Thus, if the baker’s percent for water is, say, 70%, then that means that for every 100 grams (or ounces, or whatever unit you're using) there is of flour, there are 70 grams of water. It doesn't mean that if your making a 1000 gram loaf that 70%, or 700 grams, is water—that would leave only 300 grams for the other ingredients, including the flour, which would make a very runny dough. It means, rather, that if loaf has a 1000 grams of ingredients total (including starter, water, salt, and flour), but only 500 grams of flour, then a baker's percent of 70% water would mean that you add 350 grams of water (500 x .7 = 350). This means the recipe calls for 500 grams of flour, 350 grams of water, and 150 grams of other ingredients (such as starter and salt).
Think of your recipe as a family. The mother is the center or foundation of the family and everything revolves around her. If her son weighs half of what the she weighs, then the son is 50% of the mother’s weight. If her husband weighs twice as much, then he is 200% of her weight. Flour is the mother of bread and everything else is relative to her.
So why use baker’s percentage? The primary reason is that it's easy to convert to different sizes of dough. If you're making the recipe above and decide you want to make two loaves, you can easily do the math—1000 grams of flour and 700 grams of water and 300 grams of other ingredients. Or, which is more common for me, say you only have 1200 grams of flour and you want to know how much water, salt, and starter to use with that much flour. The calculation is easy (well, relatively easy—you may have to use a calculator or spreadsheet or a smartphone app).
All of this assumes, of course, that you are measuring ingredients by weight, not by volume. So put your measuring cups away and get out your scales.
My basic, everyday recipes is this:
Water after autolyze: 5%