Adjusting a recipe for different grains

I am at the point in my bread making where I want to replace one grain in a recipe for another.  This is particularly true of replacing whole wheat with whole spelt.  The two flours do not hydrate the same (to those in the know, I suspect this is obvious, but I am new to trading grains!).  Is there a guide or some source that would give guidance as to how much water a given grain needs for a given consistency (or some other hydration guide)?  Are there certain rules of thumb to follow?  I really much prefer the flavor of whole spelt to that of whole wheat, but have many more whole wheat recipes.  Thanks for your help.