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).

-Roland