Using Grey Card for correct exposure

With the video below, you will understand why using grey card can make colour correction

http://www.youtube.com/watch?v=jxJSpxh_r90&feature=related

you may have a grey card look like the grey card show in the video, or you may have spyder cube as your grey card


Here is my try,

First
Shoot one picture,

Second, another picture including the grey card
The tools below the word "treatment" is the white balance tools, click on it!

Now, find the correct grey colour, it will drop around RGB approximate 30-35 respectively
After you click on the grey, you white balance will be change,

You will see this at the left bottom of LR3, Copy from the picture with the grey card, and paste it to the original picture without the grey card.
Crop the picture, retouch sharpness, noise, contrast, brightness.