This past weekend I rented a UGQ 20*F ZEPPELIN UNDER QUILT. Temps got down to the mid 30's and I had a couple cold spots. The second night I added an extra down blanket between the UQ and my Sparrow and had zero cold spots..

From the sounds of it I'll have to go with a lighter quilt then a -40, and just have to double up quilts when I winter hang.