We use 1 cabelas sock for a 20 ft boat when it gets really knarly out, right off the side. I generally don't put one out if the water is fairly calm. I can get down to 1.5 using just 1 of the twin 140 on the boat.

My charter captain on lake O had a 27 ft penn yan, he would always use one sock unless it was nasty out then he would go to 2 socks. I chartered with him for a good half dozen years, he taught me everything I needed to know to fish the big O and thats how he ran the socks till he retired.