how to find a "reference" lens for AF fine tuning?

[dieselnutjob]

Hi
The focus module on my Nikon Df seems to be "off", with some lenses needing to be maxxed out to -20, and all of my lenses needing fairly hi negative numbers on this body.
So I tried a bunch of my lenses on my D780 as it has the auto AF fine tune feature.
One lens was interesting because my 50mm f/1.4 AF-D lens scored a perfect zero, so as far as the D780 is concerned it's already perfect and need no tuning.
The kit 24-120 f/4 was one of my worst lenses on the D780.
So in theory if I mount the 50mm f/1.4 AF-D on my Df, zero the fine AF, and then adjust the three screws that adjust the AF module until it focuses perfectly, then this will set the Df to be the same as the D780.
I am just wondering if there is another way to test this 50mm f/1.4 to see if it really is perfect?
Because scoring a zero on the D780 just means that whatever imperfections this lens has are balanced by an equal and opposite imperfection on the D780?
is there a way to know?
thanks

 

[Needa]

I am just wondering if there is another way to test this 50mm f/1.4 to see if it really is perfect?

Testing it on hundreds of cameras might give an indication. Do you have any idea of how Nikon does testing? Have you considered sending the camera in and having it checked could be a mount problem.

My question is how do you know what the screws do. I would imagine they tip the sensor right to left and top to bottom plus moving it closer or further away.

 

[dieselnutjob]

according to comments on a YouTube video:-
Right screw: controls the leftmost part of the image
Left screw, near the lens: rightmost and bottom part of the image
Left screw, far from the lens: rightmost and top part of the image
One comment was to use a 1.5mm allen key and different one say 1/16", which is effectively the same

 

[dieselnutjob]

I took a brave pill and removed the bottom of my Df. It really wasn't difficult
The screws that are missing in this photo are the ones that need to come out:-

and this is what it looks like inside:-

these three allen head screws calibrate the focus sensor

 

[dieselnutjob]

however, I'm no longer convinced that it actually needs calibrating.
I took some more test shots:-
50mm f/1.4 AF-D set to fine-tune=0 (what my D780 says it should be):-

and 35mm f/2 AF-D finetune=-5 :-

for both shots I back button focused on the cosmetic product box which is exactly in line with the red track nibble thing, so half way between the letter B and H.
Maybe the 50mm has focused just a couple of mm behind with the H being sharper than the B but it's very little.

is it worth messing with to bring the focus just a tiny bit forward?