I always add 2 inches after I double it to take into consideration the knot. Hope that makes sense
For waist I minus two inches to account for stretching of the elastic. So for the measurements you are talking about I would make the waist 18 inches, then make the tulle min 18 inches long. That seems awfully short to me though. How old is she? I just say that because you mentioned it's for ballet, which means I would assume she is at least a toddler? For my size two tutus I measure them at least 21 to 23 inches in length of each piece of tulle.