Just posted the questions above before receiving Anjikun's message. When you click on that neutral card included in your shot, do you always get the same RGB values no matter the lighting was like? So if I can't find a spot (in non-studio shots) that's neutral, then the only resort is to play with the Temp and Tint sliders and guess at the correct balance?
If I do find a spot in my image that give me the same RGB values, then click on it with the eyedropper will get me the correct overall WB – I always thought that was correct and it's what you do with the grey card. After reading all the opinions, I am not even sure about that now.
(postscript; sorry to pile on, I see Johan also answered and slightly beat me to it; fortunately I think we are in sync.)
David, you are still reversing the process, sorry.
Let's start with the grey card. In real life (not in the image), the grey card has R/G/B values that are equal. When you take the image, the ideosynchrocies of your camera, processing, etc. will change those to some other value -- let's say 50,40,60. But they SHOULD be grey, i.e. all the same.
If you hover the dropper on them you see 50/40/60, but if you click on it to sample, lightroom will adjust the white balance so THEN they are equal -- maybe 50/50/50.
There is NO WAY to use the tools in lightroom to tell which item in the image SHOULD be grey, you just have to know. once you know, and select it, Lightroom adjusts the values so they ARE grey.
But to the grey card example: As Anjikan described it is exactly how a lot of studio work is done, but you need to be aware that in real life it hardly ever works quite that way. The problem is that the position, lighting, reflectivity, etc. of the grey card will be such that it is not, actually, grey as seen from the camera's viewpoint. So you may try this, and click on it, and while "correct" in a sense, you may get a very distasteful color as a result. It's a tool -- it is not a solution.