Does it get corrected by only the matrix value or does it get corrected by (X/Xprobe)* XprobeZ value?

That is to say that assuming the PCB is flat but not level is there any difference between only probing the 4 corners of the PCB and probing a greater number of places within the PCB perimeter?

Adding this has certainly improved the PCBs it makes. I'll stick a picture up of before and after.