Positioning MCs based on width/height

I need to place a MC on a point, as if it were positioned by the bottom right corner.

_root.createEmptyMovieClip("watermark_mc", 999);
watermark_mc.loadMovie(displayWatermark);
watermark_mc._x = 308 - watermark_mc._width
watermark_mc._y = 282 - watermark_mc._height;

Any idea why that’s not working?