I’ve been winter camping around NY state for a few seasons and am comfortable sleeping down to about 10° F, but periodically the temps drop as low as -10 F at night and I’d like to adjust my sleep system so that I can sleep more comfortably in colder weather, without paying a ton of $$ for a warmer sleeping bag. I currently use a Mountain Hardware Bishop Pass 0° F (13 F comfort), Thermarest NeoAir Xtherm pad, and a Sea to Summit Thermolite liner, along with merino wool long underwear and sometimes a fleece pants/top.

Is there a formula to determine the temp rating for a sleeping bag with a quilt layered on top? Ideally I’d like to get a synthetic down quilt that I can wrap around my bag without compressing it too much and also use it in the summer/spring for my sleep system.

Any recommendations are appreciated.