Welcome to the QB from SE Michigan! You will discover a host of very knowledge people here. Expert and new quilters and every one in between. Ask any questions you have. There is always someone who can come up with an answer. As far as batting it is pretty much you choice. Be sure to pre-wash the flannel. That being said you may want to go with something that is a low bat like Warm and Natural. It feels right, IMHO, when used with flannel. I will use a poly bat when I want more dimension to a quilt. I do want to see this rail fence when you get it done or while you are working on it. Happy quilting.