Ok, so I asked this question here a while ago, but didn't get an answer, so I figured it out myself, but then I forgot how I did it.

I want to set the z-index of several movie clips to their _y value, so that the farther down they are on the screen the farther forward they are in terms of depth. I've done this before, so I know it's possible - all I really need is a variable or property type thing associated with depth.