What does high hydration percentage on my loaf mean.
How much water in relation to the amount of flour in a recipe. The water:flour ratio.
In the "Handbook" link, at the top of the page. Section 2, Baker's Math:
Where I get the formula for rustic bread
Here are several "rustic" recipes at KA. Don't know if that's what you're looking for.
You can always type in rustic bread in the search box(top left) and probably find several threads with "rustic" loaf formulas.