The formula for metered borders I was given is: the length of the quilt side+width of border X2+6 inches. It's better too much than not enough. Although, I did make a mistake once, but fortunately had enough fabric to add a piece. I ripped it back and just added a piece - more work for not doing it right in the first place. The length of the quilt side is of course what you determine after you measure through the center etc.