The EVAL function in Calculate calls can be used to invoke a custom Sub routine on the contents of a buffer, before or after some calculation. The Sub must accept specific parameters as shown here. Typically, developers will copy this declaration from the relevant Snippet.
'Substitute OnEvalDataBuffer for your Helper Sub name
Private Sub OnEvalDataBuffer(ByVal api As FinanceRulesApi, ByVal evalName As String, ByVal eventArgs As EvalDataBufferEventArgs)
' ... do something on buffers here.
End Sub
Source: Snippet Editor: Finance - Business Rules - General
