I may need a new sleeping bag to be tent camping high on the Ammonoosuc Ravine Trail on 17 and 18 January. The average summit temperature then is -15±5C. My down bag is not rated, but it has a single-side loft of 1¾ in. which, given DougPaul’s US Army Quartermaster insulation table, correlates to 30F. I’ve had good success using it and a liner several times in Hermit Lake Shelters in early March when the temperature is (on average) 5F higher. Is it unwise to plan to use my bag/liner on the Ammy in January? If so, what bag rating do you recommend?