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Form 3 Build Platform calibration

Is there a procedure one needs to follow to ensure the build platform is parallel to the resin tank’s bottom?
I am trying to print a milling jig so it’s basically a block printed directly on the build platform.
But of course the bottom and top surfaces of this block need to be parallel.

Below is the first 4.5mm of the block (it’s hollow). Dimensions 140x140mm.

I know the height will need some fine-tuning so I’m not expecting spot-on height accuracy.
But the problem is that I am measuring 4.8mm on the left (mixer side) and 3.8mm on the right.!!

So I am guessing it’s the build platform that’s not parallel to the bottom of the tank, but I can’t believe 1mm in 140mm it’s possible (My printer works otherwise great to the extent of my knowledge)
The mixer moves left to right in the photo.

I see there’s a set screw on the right side of the BP bay at the top of the machine.

Is this meant to be adjusted by the user? Has this any influence on parallelism? It seems to be to remove side play, but I can see by the scratches to the platform that it might hinder the Z movement of the platform when securing the lever.

Update: by fidling with the set screw (by screwing until I felt some play left and unscrewing until it dissapears), the next test had a slope of 0.5mm in 140mm instead of 1mm