Pattern errata: the last row in the 16-row five braid cable is mislabled 14 instead of 15. And one detail I left off the finishing section: pick up the stitches from the holder at the top of the pocket and work on smaller needles in rib stitch for about 1/2 inch. Bind off.
Regarding the gauge mentioned for stockinette stitch - because the gauge can really vary with the different cable stitches, the best thing would be to try one repeat of the 16-row pattern across the back to see if it comes out near the size you are hoping for. You can also adjust by making the side panels of double moss stitch from fewer (or more) stitches.
Line 2 of pattern C reads:
“even rows: On wrong side row, knit as for stockinette – knit the previous row’s purl stitches and knit the previous row’s purl stitches.” Of course, it should say “knit the previous row’s purl stitches and purl the previous row’s knit stitches.”