I go about sashing completely differently. I sash each block individually, then sew the blocks together. So if I want my sashing to finish at 3" wide, I'll cut it at 2" (finished width divided by 2, then add seam allowances) and sew it on all four sides of the block. Then sew the blocks together, matching seams, and I end up with a 3" sashing where all my blocks are lined up perfectly.
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