DynamicPopulateExtender and Function Routine

My webform has 4 fields:
Food Rent Utility subTotal I'm using DynamicPopulateExtender for Food, Rent and Utility to update subTotal whenever there's a value enter.
Question #1: Do I use customScript or ServiceMethod that link to the function below
    Protected Function UpdateSubtotal(ByVal nEntry As String) As Integer
        Dim oldSubTotal = Eval("Subtotal")
        'Bind("Subtotal") = oldSubTotal + Eval(nEntry) <--- I know Bind("Subtotal") don't work because Bind is not declared
    End Function
 Question #2: How do I update Subtotal value on the webform?
 
thanks in advance
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