No I understand, I'm actually surprised I didn't think of it. Thanks a lot.Try duplicating the wall and positioning it at right angles to provide a template for the thickness. You can snap the new wall by holding down d and v and MMB clicking on the corner vertex, then just hold v and MMB click on the corresponding corner of the old wall. Then once you have the new wall in place, (theoretically you could boolean them together) but you can create a new edge loop anywhere, then snap those vertices to the new wall by holding down v and dragging on just one axis of the manipulator. I don't know if that makes sense. If I was at home I might make a screencast, but I'm just on lunch break at work.