External swf calculating stage width of what it's loading into

Hopefully that topic title is informative.

I’m doing a standard loadMovie into an empty movie clip. I’d like the external clip’s preloader to load in the center of the stage. The goal is to do what crashshop.com does. It loads a background externally, but the preloader of that external swf loads in the center of the stage. I used this in the external swf to try to do it:

_root.loader._x = _parent.Stage.width - ((_parent.Stage.width/2) - 61)
_root.loader._y = _parent.Stage.height - ((_parent.Stage.height/2) - 5)

Hoping _parent would get the file to get the info of the main stage’s width and height.

What do I use to say, “Hey, get the properties from the swf you’re loading into?”

Or would it be better to write into the main swf, “Hey, position the instance “loader” in the external swf you’re loading to load in the center of the stage.”? How would I write that?