Well, yes, sort of a formula, but not one you can follow exactly. If you place the raw edge of your folded binding against the raw edge of the quilt, stitch your preferred binding width above that, fold it down and around the quilt to the exact stitching line on the other side, then you have a double fold of fabric going up, down, and up again - so your fabric would need to be 6 times as wide as your binding width. Where this breaks down is that you also need to account for the thickness of your quilt top, so you add something for that. And you probably want the binding to come up a little higher on the side where you hand stitch it down, so you want to account for that. And if you want a wider binding (such as 1/2") you may decide not to start it out even with the quilt edge (I do .5" binding on larger quilts, and start the binding edge 1/4" above the quilt edge), so you have to allow for that. And although everyone says to make your binding full, some people make it a lot tighter than others. So... a formula can get you started, but you need to figure out what is right for you and your quilt.