A multiple of 8 + 1 means any multiple of 8, plus one extra stitch. So to get closest to 91sts you'd have to do 8sts x 11repeats = 88sts, plus one extra stitch which would be 89 stitches. It would actually probabl work pretty well because that's only 2 stitches off from what your back count should be, so just make the first and last stitch of the back stockinette (knit on rs rows, purl on ws rows) and the 89 lace stitches as the rest of the back stitches. Hope that helps.
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