Measuring a grayscale ramp pattern, full-screen, to get a luminance to determine how to setup your measurement patterns is REALLY REALLY REALLY difficult (and frankly, I'm surprised a calibration-related professional would even suggest trying to measure a full-screen ramp pattern at all because of the extreme level of difficulty
Actually it's very easy with a K10-A, which it the probe we use for plasmas, and the reason for suggesting it is we use it all the time with very, very good results in professional post-production operations where they have not yet replaced the plasmas they purchased before the issues with ABL were fully understood...
I would never suggest something we do not use as an integral part of our calibration process.
But, as I keep saying, if you have a process that works for you, stick with it.
With plasmas there really is little that is truly right or wrong.