I didn't know there was a rule of thumb. I like borders on my quilts and if I still need to make it either longer or wider, I'll add an extra section of sashing/border to fill in such as background fabric on the outer edges with colored blocks between background fabric in the middle like the film strip look. I'm planning on a quilt for my bed with very long side borders as I want a plain section to add a scene along the side.........oriental for this one. The top and bottom borders will not be that wide as I won't need it but there will be a border to tuck. I have a 4-poster antique bed so it won't show anyway.