I'd say the best way to think of it is this: the "left" or "right" measurement is how far to the left or right of center the steroscopic sweet spot is. Roughly speaking, if your head is 20 cm to the left of the centerline of your car, the best results will be around 0.6 msec left.

I think the best way to tune this setting is to sit in the car with music playing, close your eyes and try to hear where the center of the music is... when it is in your head, bingo!

Hope that helps.