Make pop ups appear in middle of the screen.

Can anyone please tell me how to make my pop up appear in the middle/center of the screen?

Right now my code is

<a href="#" onClick="javascript:window.open('webpage.htm', '','width=550,height=550,status=0,scrollbars=1,resizable=1')">pop it up</a>

Thanks

http://www.eak.homeip.net/portfolio/newwindowgen.php