Subject: 12. What is meant by % hydration of a dough
"Bakers Formulae" are based on the weight of flour which is assigned 100%. Any other material being added is expressed as a percentage of this. Thus water may be at 55% to 60 to 65% of the the flour. If you think in metric terms it is very easy each 1000 grams (1Kg) of flour would need 600 grams of water for 60% hydration etc, similarly salt may be added to 1-2% etc etc. So a bakers % is actually a very slippery definition and not "correct" in scientific terms but they understand each other.
The % hydration matters both when you feed/build your starter and in the final dough. Studies show the maximum acid is built at 90% hydration (during feeding/building).
Sourdough QA|Sourdough FAQs|Darrell Greenwood's Home Page
Links changed to carlsfriends.net 10/8/16