Determine Your Flour's Hydration
from The Bread Code

The amount of water your flour will absorb depends on the type of flour and the percentage of protein in the flour. To test this, get 5 bowls and put 40 grams of flour in each. Add:

• 26 grams of water for 65% hydration
• 28 grams for 70%
• 30 grams for 75%
• 32 grams for 80%
• 34 grams for 85%.

With your fingers stir each until well mixed and wait for 30 minutes. Basically just doing an autolyse. After 30 minutes, he found for his flour:

• 65% hydration - dough has built up some gluten and stretches without tearing, but he thought the dough too stiff. A stiff dough would yield a more dense crumb, not as fluffy
• 70% hydration - Good structure forming, stretches more easily than 65%
• 75% hydration - Holds its shape really well. Also excellent. This one is a safer bet than the 80%
• 80% hydration - Also looks good. Would need good technique developing the gluten, but looks really good.
• 85% hydration - Dough is tearing too much, is very sticky. Looks kind of gloppy. He rejects this one.