Buy a bag that is rated at least 10 degrees lower than what you expect outside temps to be
Read the fine print. A few years ago I looked at a bag rated to 20. In the fine print it said that their ratings were for the temperature that you would keep you alive, not comfortable.
Two inches of loft (each side) will take a well fed adult male to about freezing with comfortable sleep. Warm sleepers 10F less, most females 10F higher. North Face (who's ratings are trustworthy) rate the Cat's Meow at 20F, the women's model at 30F.
Every 10 degrees lower will require slightly less than another inch of loft(per side). Once you know this you can eyeball a sleeping bag and know about how low you can sleep in it.
