Access value from Binding

[Joel]

I have a report where there are a set of known values which are dynamically set.  The detail section shows numbers that are used for comparative purposes.  I am looking to create a binding or some other method that will evaluate the difference between the two values.

Example: 

Design surface:
In groupheadersection1 I have a textbox  txt1
In detailsection1  I have a textbox txt2 with a value of =fields.someval
In detailsection1 I have a textbox  txt3 which needs to show the difference between txt1.value and txt2.value

Code:

Partial Public Class tstClass
    Inherits Telerik.Reporting.Report
    Public Sub New(ByVal _ds As DataSet)
        InitializeComponent()
        txt1.Value = _ds.Tables(0).Rows(0).Item(0)
        Me.DataSource = _ds.Tables(1)
    End Sub

Accessing the value via expressions seems to fail.  I created a binding, to a public shared function, but of course the shared function cannot access txt1.

Any ideas on how to get the value for txt1.value-txt2.value??