I'd agree with that 5*F estimate, Glenn!

In other words, the Marmot Helium would be a 20* bag, but be sure to get the model with the full-length zipper so you can ventilate it in warm weather.

The Western Mountaineering Ultralight (if not too narrow for you) will go down lower than its 20* rating; it's a very conservatively rated bag. If I start getting cold, I just snug up the draft collar a little tighter, and I'm cozy warm! And, as mentioned, I'm a really cold sleeper. Their Alpinlite is a wider version of the same bag.