The problem with plasmas is you actually need to match the patch size to the impact ABL has on that particular display.
The problem is that when ABL kicks in it is in a different point based on the scene brightness/contrast for different displays.
So, if the patch size is too small you will calibrate for a different level of ABL interaction compared to real images.
But, then, there are no guarantees on the real images you will be looking at...
And that means there are no real answers to the correct patch size.
The best will be to profile the display and measure the ABL effect on different patch sizes and chose the best compromise for that particular display.
Rule of thumb - the more aggressive the ABL the larger patch size you actually need - up to a point.
And yes, that is counter intuitive to what you might expect....