Match the lighting
Place the reference where the subject's light can reach it without an unrelated reflection. A reference measured in different light may answer the wrong question. Follow the card and camera instructions for the particular method you use.
Separate exposure and colour
A brightness reference and a colour-balancing reference have different roles. Decide whether the task is exposure measurement, white balance, or later comparison rather than treating them as interchangeable operations.
Verify the result
Inspect important tones and colour relationships in the photograph itself. Keep a reference frame alongside the sequence so you can compare later edits with the capture conditions.