Just wondering why you would not use the white balance card to set a custom white balance and then there would be no need to adjust from a reference shot?
I am a little unclear on the scope of your question, so I'll answer it broadly and that way probably hit on what your specific question was.
Color temperature (white balance) is an attribute of the light falling on the scene being photographed. If the light changes, then the color temperature changes. If a sunny day becomes overcast, or morning becomes afternoon, or someone turns on a lamp, the color temperature of the light changes. It is not a value that can be preserved as an eternal constant within the camera.
Automatic white balance settings attempt to arrive at an estimation of the color temperature. They usually do a decent enough job, but certain situations, particularly indoor scenes that may contain a mixture of different light sources, confound them. A white balance card is a card that is 18% gray. By using it as a reference in the scene, it provides an exact control which you can use within your editor to specify the white balance.
In theory I could use the card to directly measure the white balance and create a preset value that way. I shoot with a Nikon D700 and it support this. Your camera's feature set may be different. Assuming your camera supports this capability, you can read about that process in your camera manual.
I find it's easier to just set the card down and grab a quick photo of it. Then after I ingest I use the WB selector tool in Lightroom to specify the gray card as a reference point and use it to derive the WB for the scene. I copy that value across the remainder of the photos in the shoot and I'm good to go.
On a semi-related note, I do usually use a custom preset white balance setting. It's called UniWB. It causes the RGB histograms to more closely reflect the light actually being captured by the camera. The resulting photos have a strong greenish cast that is corrected when I adjust WB in post.
