The baker's percentage isn't a true percentage, but a way of expressing the formula that can be scaled up or down to any amount. The ingredients are shown as a percent of the weight of the flour.

The percentages for a very simple recipe such as Floyd's Daily Bread showing the sub-percentages of each step would be:

Poolish

150 g flour = 100%

236 g water = 157%

0.8 g yeast = 0.0053%

Note that for home baking quantites most people measure the yeast by volume because amounts that small are difficult to weigh and it is quite easy to multiply "1/4 tsp" by 2, 3, 4, or whatever

Final Dough

454 g flour = 100%

284 g water (10 oz) = 63%

3.2 g yeast = .007%

12 g salt = 2.6%

386 g poolish = 85%

Total

604 g flour = 100%

520 g water = 86%

4 g yeast = 0.66%

12 g salt = 2%

The 86% water in the total recipe is also known as the hydration percentage or just "the hydration" (86 is a high hydration!).

To make good use of baker's percentages you have to weigh your ingredients (whether you use metric, English, or US weights doesn't matter although most people find metric the easiest) and brush up on your 7th grade arithmetic (especially ratios). But once you do that you find that it is easy to read a new recipe and get a sense of how it will turn out, and also to scale recipes up and down.

One caution is that like everything in baking various regions, cultures, cookbook authors, web posters etc have slightly different methods of calculating the percents. And of course all will be quite firm that their method is THE CORRECT one ;-). So you have to do the arithmetic once or twice for each new cookbook to verify how that author is presenting the numbers.

HTH. If anyone spots an errors please post them as a new comment, not a reply to this comment, so that I will still be able to edit/fix.

sPh