Most people miss that there is a primary exposure point in an image rather than an overall correct exposure for a whole scene. The reflectivity of the various parts of a scene make them record relative to the primary exposure point. The key to proper exposure therefore becomes a matter of finding the primary exposure point. This can be easy, and it can be complicated.
The primary exposure point is the one that will record our primary subject at a level we find appropriate for the image we want to create. In a portrait, for example, the overall skin tones should be recorded appropriate to the look we want for the portrait. All of the other tones in the image will record relative to that decision because of the differences in their reflectivity compared to the subject reflectivity.
The easy way to set a primary exposure is with a hand-held incident light meter reading taken at the subject location with the flat base of the hemisphere dome parallel to the primary plane of the subject which usually coincides with the plane of the camera back as well. That will work well over 95% of the time for a normal photograph. It is when creative results are desired that the camera settings will need to be adjusted to suit our purposes.
The hard way to set a primary exposure is with a reflected reading with a TTL metering system. While that may work with a fairly high percentage of success in normal lighting circumstances, it can fail dramatically in contrasty light, backlighting, or where subject reflectance is just not average. Snow and beach scenes are notable problems as the higher than average reflectivity of the primary areas of the scene cause the meter reading to incorrectly record the primary subject. Similarly, subjects like brides in white dresses and grooms in black tuxedos will fool reflected readings.
This is why you see professional photographers carrying hand-held meters. They are also the best way to evaluate differences in lighting ratios. You can seriously improve your exposure success by using a hand-held light meter. With the instant feedback of digital, you can also learn to use the histogram as a support tool for evaluating subject exposure. It is not as easy as a meter, but better than trusting a reflected reading.