Whatever seam width you are going to sew the binding on with X 6 would give the size of binding to cut.
I think you would only want to use say 1/2 binding if you have a wider border. You need to stay 1/4 if you don't have a border and you have any point on the outside of your quilt. Could do the 3/8 without a border only if no point in on the sides.