Chrome's had a issue with this for a bit - popup open position in chrome.
Also How do I get JavaScript to open a popup window on the current monitor.
You should be able to use the screen left position though to work out the math. I did something like this to get the window to appear to the left of the browser window. Again, Chrome may be a issue.
leftPosition = (window.screenLeft != undefined ? window.screenLeft : window.screenX) - width;
topPosition = 0;
window.open(url, "Title", "status=no,height=" + height + ",width=" + width + ",resizable=yes,left=" + leftPosition + ",top=" + topPosition + ",screenX=" + leftPosition + ",screenY=" + topPosition + ",toolbar=no,menubar=no,scrollbars=no,location=no,directories=no");