What’s all that jazz in the 2nd line about? I can’t parse it: does it have something to do with CC Particle emitters being registered at the center of the comp instead of the upper left? (I think I’m close, but I don’t get it….)
That’s because the Particle World Producer property expects its X coordinate (in this example) to be a percentage of comp width relative to the comp center. So the the second line of the expression calculates how far the null’s x position is from the x position of the comp center and divides that by the comp width to get a normalized value between -.5 and +.5.