Center a movie clip that is included in a flash popup

So I am creating a photo gallery and in the process I have created a button that links to a movieclip and plays it to appear as a popup window of the larger picture.

on(release) {
attachMovie(“gallery_mc”, “window”, 1);
window._x = 250;
window._y = 150;
}

I then included a close button that simply posts

on(release) {
this.removeMovieClip(window);
}

I am curious as how to make the movie clip popup centered in the screen as it is now popping up in the upper left hand side of the flash movie. Any suggestions? Thanks.

Bryant

PS, Attached is a screenshot of the positioning.