What I meant was something like this:
Dim mainNavigator As XPathNavigator = Me.CreateNavigator()
Dim codeInfoNavigator As XPathNavigator = Me.DataSources("Get_Existing_Engagement_Code_info").CreateNavigator().SelectSingleNode("/dfs:myFields/dfs:dataFields/ns1:GetColumnsXMLStrQueryResponse/ns1:GetColumnsXMLStrQueryResult/Rows/row")
once you have that, the code is more concise and efficient.
mainNavigator.SelectSingleNode("/my:myFields/my:Date_Letter", Me.NamespaceManager).SetValue(codeInfoNavigator.SelectSingleNode("BudgetCost").Value)
I think there's probably very little difference in speed versus setting a field value in code vs. in rules, but at Qdabra it's our policy to use rules rather than code whenever possible because it is more straightforward, makes the form easier to maintain, and doesn't require coding experience to make modifications.