Short answer: for those seasons, get a 0 deg AND an appropriate pad.
Long answer: If you can find a bag/company that EN rates their bags you'll get a better picture of what you're working with. If not, Stid's advice is spot on, IMO.
Hunting CO 1st rifle season this past year, I was in a Marmot Couloir 0 deg (down), EN rated at -3F or somewhere in that neighborhood and I slept most of the night with it partly unzipped. When I did zip it, I didn't have to cinch down around my face much at all. Conversely, on the trip before that I was in a North Face Elkhorn 0 deg synthetic and I had that thing zipped up and cinched down as tight as I could, still couldn't get a good night's sleep. The difference? Not so much the bag above me as the pad under me. The first time all I had was a Thermarest Ridge Rest and the second time I had an Exped Downmat 9. Not even comparable for heat or comfort. The Exped is a lot heavier but I'll suck it up every time for the quality of sleep I get with that thing.