As an ISF calibrator I work on many displays calibrated by others because they hired someone else with lesser skill or tools. These charts show the difference in the quality of white point calibration between a quality job and a good one.
When the gray scale is perfect the mix of red, green and blue in white should be at 100% from 10 percent signal level to 100%.
The poor gray scale shown above cast a strong yellow cast to the image which never looks good. The proper gray scale below rendered the image with a white balance that made the lighting of scenes look very natural. As with any other service the quality of the results will depend on the skill, desire and tools that the person performing it brings to the job.