A cable pattern gauge will always be smaller than a stockinette gauge. If you have a 6 st cable with 2 sts on either side of it for the 10 sts/inch, that may very well come out to 7-8 sts in plain stockinette. Sooo, do it the other way - CO for the cable gauge, work 3" or so, then garter a couple rows and switch to plain stockinette for 3". You would still compare them.
sue- knitting heretic