I got the Feathered Friends nano 20 I think it was, also they called it that but I don't think it was a specific 20 degree rating. I'm good at 20 degrees in that without much on, with my base layers on I'm good in the single digits then I use the down jacket either on or extra around my head and I've been good down to 0 sleeping comfortably. A little trick if your not tight on weight just pack a few hand warmers, especially when your testing out for the first time. It's like a little heater in there, will warm you right up. I did not use hand warmers at the temps I mentioned but I was glad I packed some when I had my 35 degree Western Mountaineering and they were calling for 17 degrees!
When on a multi day hike with a down bag you want to take it out and fluff it up as soon as you get to camp, and repeat fluffing, turning it and even give it some sun if you have the chance. Just your body heat with it inside the tent will help a lot as long as it isn't too bad. Sometimes my bag looked pretty wet and droopy when I took it out but taking care of it like mentioned above worked pretty quick and well.