Getting the coordinates where the browser screen is

I would like to open a new window using javascript and window.open but my idea is to open that window, a small one, on the horizontal and vertical center of the current window.

For simplicity, suppose I have a screen that is 1024x768 and I want to display a 200x100 little window on front of that. So, I need that little window to be centered at (512,384) that is the center of (1024,768).

Something like this:

I am using this code to discover the current coordinates:

var x = window.screenX;
var y = window.screenY;
... proceed to window.open...

but this produces a window that is like this

and because I have 2 monitors, when the browser is on monitor 2 this produces a little window positioned on the upper corner of the first monitor.

Is there a way to get the right coordinates?