Cables pull in the width, how much depends on if it's a simple singe cable, a double or what. You'd best make a sample - CO enough for the cable repeat, or 2 if there's 2 side by sid and add several sts to each side for the background. Work a couple of repeats of it, should have at least 2 or 3 crossings, then switch to stockinette stitch and work to the same length as the cable part. Wet it and lay flat to dry, check the stitches over 4" or even 5 if it's wide enough and see what the difference is. Say 5 sts per inch in the stockinette vs 5½ in the cable maybe.
sue- knitting heretic