When you hang the rain fly (or tarp) independent of the hammock do you use a ridgeline running the long axis of the fly and tied off to the trees to support the rain fly, or do you just tie the ends of the rain fly to the trees? I can see the advantages of hanging the fly independent of the hammock and I'm trying to figure out the best way to do it. I have concerns that the rain fly might sag too much in the middle without a ridgeline for support.