As I recall, the temperature rating is supposed to indicate at the lowest temperature at which a bag will let you sleep "comfortably" under "normal" conditions (say, wearing minimal clothing, like shorts and a t-shirt, on a windless night in the open on a closed-cell foam sleeping pad. Or something like that.)

The problem in the US is that there are no hard and fast standards for "comfortable" or "normal", and each manufacturer gets to self-rate their bags. That leads to a system of "how low is that bag rated?" "How low are you looking for?"

There is a more scientific method used in Europe - it's designated by an EN rating, and bags from different makers with the same rating will pretty much keep you the same amount of warm.

So, absent an EN rating for your bag, it comes down to who made your bag. If it's Western Mountaineering or Feathered Friends, a 40 degree bag may still be warm at 35. Marmot will be pretty much dead on; Kelty, REI, and Campmor may also be pretty accurate, though I'd probably allow 5 degrees - call them a 45 degree bag. Off brands might very well mean a 40 degree bag is a 50 degree bag.

You can lower the temperature in which you can use a bag several ways. First, sleeping in a good tent will add perhaps 5 degrees to the rating. Wear long johns to bed, and you may get another 5 degrees. Now that 40 degree bag may let you sleep warm at 30. Add a pile or down jacket and pants, gloves, a balaclava, and maybe a stocking cap, and you may be able to get down near 20.

I say "may" because now two other factors come into play: bag size and sleeping pad. You want a bag to fit fairly snugly; if it's too loose, you end up with too much air to heat, and that 40 degree bag may only work like a 45 degree bag. However, if you're trying to push the limits of the bag by wearing a down or pile jacket and pants, you may end up being larger than the space inside the bag. If that happens, you end up compressing the bag's insulation against the outer shell, and lose warmth. (think Goldilocks: this bag's too loose, this bag's too tight, I need one that's just right.) The sleeping pad will not make a bag warmer; however, on very cold ground (say, in the winter or on snow) a pad that has too low an R rating will let the cold seep up from below and chill you, even though you have the right bag otherwise.

Really simple, huh?

Let us know your make and model of bag, and what pad you'll be using, and we might be able to give more specific advice.