Okay, I know that you can use a spot meter to place values, but a center weighted meter pretty much evaluates the whole scene and averages for the value at the center. So with that information, how do I learn how much compensation I must apply for certain situations?

Example: backlit portrait, I know I'll need to open up (and with a spot meter, I would meter the face and open up 1 stop), but how much/how do I determine how much?