Danno,
I have been using the ExpoDisc for about four years now. I am absolutely a believer in this this product. I find it can nail the white balance in mixed lighting situations better any other tool in my opinion. While are cameras do have presets for fluorescent, tungsten, etc… those lights and their colors can always very from age, manufacturer, or a whole host of other things.
This tool also serves as an incident meter for portrait work to nail exposure.
I use to build Jeeps and this product is very much like re-gearing the axles on the Jeep. It’s not flashy doing gears like adding big tires or some cool body armor, but at its heart, gears are an absolutely valuable workhorse tool that has big benefits but no flash. That is what the ExpoDisc is to me – a no flash workhorse.
Here is a short video showing it used as an incident meter and setting WB for Canon:
https://youtu.be/DHWn3MQ0YYY
The incident meter part applies to Nikons as well, but setting white balance is much easier. For Nikon you set WB to “Pre” then press until the top display of you D7200 start flashing “Pre.” Then you put the ExpoDisc over the lens, hold the camera in front of your subject while pointing the camera back to where you will be shooting, and snap the shutter. The top display will then flash “Good” if the camera got a good reading. When I take a reading I set the camera to the ISO I want and aperture priority with no exposure compensation. That way the shutter will be open as long as needed for a good sample.
Hope this all helps. Feel free to ask me if you have any other questions.