Positioning PopupControlExtender control

Hello,

I am attempting to center the position of a Ajax Popup control but having some difficulty. I've tried changing the OffSet properties of the Ajax PopupControlExtender via JavaScript but to no avail. Here's sample of the code:

// JavaScript
// center popup control                
        var left = (screen.width / 2) - (iWidth / 2);
        var top = (screen.height / 2) - (iHeight / 2);$find('NonModalPopup').OffsetX = Math.round(left);$find('NonModalPopup').OffsetY = Math.round(top);


//HTML
<cc1:PopupControlExtender ID="HorizontalMenu_PopupControlExtender" 
        runat="server" BehaviorID="NonModalPopup" DynamicServicePath="" Enabled="True" 
        ExtenderControlID="" TargetControlID="AccessKeyPanel" 
        PopupControlID="PanelNonModal" ></cc1:PopupControlExtender><asp:Panel ID="PanelNonModal" runat="server" CssClass="modalPopup" 
        style="display:none;"   BackColor="Control" 
        BorderStyle="Ridge" BorderWidth="3px"><div class="header" runat="server" id="divMasterPageTitleBar">            
         TBD   <a onclick="AjaxControlToolkit.PopupControlBehavior.__VisiblePopup.hidePopup(); return false;" title="Close" class="closeDialog" id="ALink_NonModal">X</a></div><iframe id="IFrameNonModal" src="" runat="server"></iframe><div class="footer" style="display:none"><input id="btnOk_nonModal" type="button" value="Done" /><input id="btnCancel_nonModal" type="button" value="Cancel" /></div></asp:Panel>

Any help is appreciated.