Another handy thing to know

 

String xpath = 
  "sum(/ns1:Invoice/ns1:InvoiceDetail/ns1:Lines/ns1:Line/ns1:AmountMatched)";
int amountMatched = 
  Int32.Parse(this.MainDataSource.CreateNavigator().Evaluate(xpath, this.NamespaceManager).ToString);
The above code shows how to evaluate any valid xpath expression so that you can calculate values through code-behind, very useful. As it currently looks like default values/formulas are only calculated when a field value changes and not at form load.

 It is worth noting that you can use the Evaluate method on any XPathNavigator object