When it's 50 degrees outside in the middle of a windy night, personally, I'd like the same coverage on my top quilt as I would from a quilt when it's 20 degrees on a windy night, so I would order the same length you would for a cooler-weather top quilt. You're already saving weight by ordering an ultralight 50-degree top quilt; for my own use, I don't see the point of being so stingy with the length that it becomes less practical or less effective. You can get away with having a very short underquilt, especially in warmer weather, but a top quilt that is even a couple of inches too short can really prove annoying. Yes, you may be folding back the quilt to vent your torso on a 60-degree night, but you'll benefit from every inch of length on a night when you unexpectedly push the rating...