We have developed a process of using a grey ramp image to calculate the ideal patch size for any given plasma, as different display have different requirements.
Use a Grey-scale Ramp image - something like the attached.
Then measure the overall brightness of the screen (with the probe covering all the screen - moving back and forward to get the peak reading).
Just looking for Luma not colour readings.
Then make a calibration patch window with 50% white and size it to get a similar Luma reading with the probe in the same place.
Use that patch size for the profiling (moving the probe to the correct distance for the patch size).
There is some discussion the best profiling to use - Quick Profile or a Full Profile.
Both can work, but after a LOT of testing we do now see benefit with a Full 17 point cube.
Mob Boss at Light Illusion