You can draw some general conclusions on a quilt's effectiveness based on loft, and this may help you compare one quilt/bag to another...but that's all. So many other factors affect the true temp someone can take a quilt to, and some of them won't even be constant for a person. Hydration level, that day's exertion level, whether you're hungry, how long it's been since you last ate complex carbs, humidity level, wind speed, dew point, price of tea in Thailand, etc.

That's why it's difficult for a manufacturer to give a solid temp rating.