That screw is a stop on the support clamp. It is not really for adjustment of the file.1:You set the clamp to the height you want it. Now the the arm can swivel on it so that you can swing it out of the way.
2:With the file set to the required angle you use the adjusting screw to swing the arm til the wheel just touches the file.3:Lock the nut on the screw.
Now when you swing the arm away and back, it will return to the same position.
Repeat steps 2 and 3 when you change the file.
Paul K. Dickman