A long time ago a guide told me that if you add a liner to your sleeping bag, it's the equivalent roughly of adding 10 degrees (Fahrenheit) temperature rating to your bag. Wonder if anyone has tried this out in conditions where you really needed the extra 10 degrees and it worked for you? Debating between a 20 degree and a 10 degree sleeping bag, and wondering if this trick might help me extend the season of a 20 degree bag.