Instead of drawing a line, you can use blue painter's tape on the quilt top to keep your lines straight. Much easier and faster! The tape is reusable too. Would work well in the PVC type frame.
Edit: For designs in the center areas, I would find it easier to switch to a lap hoop -- the type that allows you to easily make adjustments in the positioning. Mine swivels on a wooden ball but is no longer sold; the closest I have seen to it is the Grace lap hoop.