I LOVE my 40° Incubator +2oz overfill. All the backpacking I've done with it so far had temps between 35° and 50°F and it was perfect. I think it was close to freezing one night testing in my back yard. Around 4am I pulled in a 18"x24" piece of car windshield sun shade to cover some of my rear and back. I didn't really need it, but it sure helped me get right back to sleep. I recommend carrying one for some extra peace of mind when they overshoot the forecast by 10°!
So, OK near freezing...FOR ME. It's completely up to your body, clothing, and many other factors. Can anyone help out codya95 any further? Anyone get into the 20's without a lot of supplemental insulation? Of course at that point I would just take a 20° Phoenix or Incubator.