I have a little trick that I came up with for mounting rings which have the top and bottom 2 screw per side 'sandwich' style like the one pictured in the video above.
Anyhow its obviously ideal get the gaps between the top and bottom rings perfectly even on all 4 sides. However to wind up with them all even requires alot of neck stretching. I have a plastic hotel room key card that I cut into 4 strips to act as spacers. I slightly screw down the rings until I can put a strip in each gap hanging downwards by its own leverage. As you tighten the screws and close the gap, the strip will begin to move upwards closer to horizontal. You can adjust each side of the rings to get the same even angle of tilt on all the hanging strips and then tighten the screws until the pressure holds the strips horizontally. At this point all the gaps are perfectly even and then I tighten down a bit more until I can barely pull out the strips. At this point you can go back and forth tightening the screws and wind up with 4 perfectly matching gaps. This technique works really well for obsessive compulsive types.
Heres a pic:
Ingenious!