I'm looking to replace a very old sleeping bag that has lost all it's warmth. Given that I haven't shopped for a winter bag for a looooong time, what temp rating should I look for? Usage would primarily over a 2-3 day weekend, not expedition quality. I already have a WM Highlite 35. If I nest it with another WM bag, say 10, what would my temp rating become. I'd like both cheap and expensive ideas.