I have been down in the single digits in my nx250 with a DIY synthetic converted sleeping bag. I made a top and bottom quilt. The keys as I see it are to dart the sides of the UQ to form a bathtub shape and both elastic on the ends with channels sewn all around for shock cord. With the TQ I put in a foot box and wings (think of segmented pad extenders) so no matter how I lie or spin around I am covered and warm. I wear fleece pants, hoodie and a light knit poly cap. I also have magic socks that are very warm but when i fall fast asleep they go on wonderful adventures and end up in the strangest places, but never on my feet.