Popup In Center Screen

Hi Experts ! 
Dim url As String = "Frm_Show_Iqama.aspx?EMPNO=" & msg_EMPNO.Text Dim sb As New StringBuilder() sb.Append("<script type = 'text/javascript'>") sb.Append("window.open('") sb.Append(url) sb.Append("','_blank','location=no,status=0,scrollbars=1,resizable=no,top=200,left=250,width=325,height=220');") sb.Append("</script>") ClientScript.RegisterStartupScript(Me.GetType(), _ "script", sb.ToString()) I am Using Above Script To Open Popup From Code Behind. In ASP.NET (VB)
But I Can not Able To Make It On Center. . . 
Can Anyone Help Me Fix Script To Make Popup In Center Screen.

Thanks.