I have a couple books just on pieced borders. What they all recommend is 'filler borders' or 'coping borders'. You make your top, sash the blocks with the same width sashing top & bottom, but don't put any sashing on the outer edges. Measure your finished top, then add whatever width filler border you need to make up the difference between the top & your pieced sashing length. Unless your quilt is square or the math comes out exactly, the side fillers & the top & bottom fillers can be different widths. Because it's a border, it looks fine.
The other thing that's nice about this system is you can make the top w/o the filler borders, piece your pieced border and measure it then cut the fillers so the pieced border fits exactly.
Last edited by PaperPrincess; 01-12-2015 at 05:27 AM.