I don't have the book, Angela. If no one who has the book replies, then try this:
I'm guessing it's the primary liquid in the muffin recipe. Put your dry ingredients together. Then add the yogurt 1/4 cup at a time and mix lightly until it gets to the consistency of your regular BB muffin recipe. I'd guess at 8 oz for the batch.
Don't overmix. Muffins get nasty rubbery if you do.
Mary
Mary SW: 169.0 (6/18/08) CW: 151.0 # lost: 18.0 Goal: Size Perfect or 135 whichever comes first!