Set ASP.Net AJAX ModalPopupExtender position in VB.Net

this is the dropdownlist with autopostback:

 <asp:DropDownList ID="DropDownList5" runat="server" Height="16px" Width="324px"  onchange ="setPopup()" AutoPostBack="True">
 </asp:DropDownList>

and the function in javascript :

<script type="text/javascript">
    function setPopup() {
        var pop = $find("mpe");
        var scr = screen.width;
        if (pop != null) {            
           pop.set_X(scr * 0.7);
           pop.set_Y(scr * 0.05);             
        }
    }
</script>

and the modalpopup :

<ajaxToolkit:ModalPopupExtender ID="mpe" runat="server"
 TargetControlId="Button8" PopupControlID="ModalPanel2" 
 OkControlID="Button6" BackgroundCssClass="modalBackground2" ClientIDMode="Static"/>

 and with the button work fine, with the dropdownlist no