I would like to adapt a recipe that calls for 80% hydration using whole wheat flour.
What should be the % hydration if I use whole spealt flour?
60% would probably be a good starting point. The key though, is to hydrate and mix the dough to the consistency as described in the recipe; ie, sticky, tacky, soft, firm, etc.
I just made a loaf where I substituted half the flour with spelt, but I didn't have to reduce the water. In fact, I had to add a little extra to get to the needed consistency.