I haven't worked with flannel and there are a few variables to the answer. How wide do you want the finished binding?
What seam allowance do you use for sewing the binding onto the quilt? 1/4, 3/8 or somewhere in between?
I suggest doing a small sample size. Cut a sample binding at 3 inches and go from there. Easy to do so you get exactly what you want.
PS: I always read your name as RV's fan meaning you travel in an RV. LOL!