For a pretty good view of baking formulas and other technical data, see if your school library has a copy of Daniel t. DiMuzio's Bread Baking: An Artisan's Perspective."
Meanwhile, here's the simple version: Formulas are valuable because the provide an easy way for bakers to multiply or divide a "recipe." You already know the key part: that flour is always expressed as 100% by weight.
So, to keep the math easy, let's say you have a formula that calls for:
Pate fermentee: 168.4%
Bread flour: 82.4%
Whole wheat flour: 15.8 %
and you'll be starting with a 100 pound sack of flour. Just multiply everything to establish the other needs. Thus:
Pate fermentee=168 pounds
Bread flour=82.4 pounds
What if you only have a 50 pound bag? Just do the same math:
Bread flour=82.4%, therefore .824 x 50=41.2 pounds.