I have been photographing people interacting in a cube (with the side facing the camera removed). It takes me quite some time to get my camera+tripod set up correctly to find the exact centre of the cube in all three dimensions (sort of analogous to yaw, pitch and roll). So far I have been doing this by eye, but this 'technique' is not exact enough and necessitates subsequent correction of horizontals and verticals in PS. How do I achieve perfectly symmetrical perspective in foolproof fashion? Any tips gratefully received.
Can you (temporarily) mount a mirror (tile) on the rear cube face?
That will allow to center your lens' reflected image in the center of your viewfinder image. When you mount the mirror at the exact center of the far cube side (using 2 diagonal lines to find that center), your camera/lens will need to be positioned exactly symmetrical in height and width, and square to the rear cube face to see that mirror image of your lens.
Otherwise, you may need to resort to very precise distance measurements, from entrance pupil position to corners, e.g. with a laser distance finder.