sounds like a good plan, but please be sure and check the info on your batting package. That will tell you the farthest apart your quilting lines can be. Just double check that a single line of quilting down the zigzag will provide enough coverage. You don't want your beautiful quilt to get ruined the first time you launder it.
Also, did you prewash the flannel? Flannel shrinks a lot more than quilting cotton and could also pull your quilt out of square.