Is this card used for digital photography or only film?
The Sekonic Folding Stand-up 18% Gray Card can be used for both digital and film photography. However, you will probably not need it for digital photography since you can easily use bracketing and - depending the digital camera model - even auto-bracketing and view the results immediately afterwards on the digital camera screen. In the time it will take to utilize the Sekonic Folding Stand-up 18% Gray Card you will probably have taken all the bracketing shots you need to with your digital camera and thus have the photos already. For digital photography therefore I would use bracketing since it is faster and saves you from carry unnecessary equipment around with you.
If I may give a word of advice, unless you are squeezed for memory on your digital camera's memory card don't delete photos from the memory card based on how they look on the digital camera's tiny screen. Always check the photos on a big screen too because sometimes even just a part of a photo may be worthwhile saving.
With film photography it is another matter. Since film photography is more expensive than digital photography and you will see the results first after the film has been developed, using the Sekonic Folding Stand-up 18% Gray Card can be worthwhile. Nevertheless, also with film photography I use sometimes bracketing especially in high contrast environments.
I hope this helps a little.