I have a Template Column created where I want to compute the percentage difference
between two other (bound) columns. Using the code below, I can compute the
column value. However, I am not able to sort on the column (the SortExpression="Variance"
is incorrect). I need to know what
to set the SortExpression=? Is this even possible?

<asp:TemplateColumn SortExpression="Variance" HeaderText="Variance (%)">
<ItemTemplate>
<asp:Label>
<%# getVariancePercentage (
(int) DataBinder.Eval(Container.DataItem, "actl_qty"),
(int) DataBinder.Eval(Container.DataItem, "expt_qty")) %>
</asp:Label>
</ItemTemplate>
</asp:TemplateColumn>

Thanks! Matt