Private Sub TextBox1_AfterUpdate()
Dim PopulatedFields As Integer
PopulateFields = 0
If Not IsNull(Me.TextBox1) Then PopulatedFields = PopulatedFields + 1
If Not IsNull(Me.TextBox2) Then PopulatedFields = PopulatedFields + 1
If Not IsNull(Me.TextBox3) Then PopulatedFields = PopulatedFields + 1
Me.TotalTextBox = (Nz(Me.TextBox1, 0) + Nz(Me.TextBox2, 0) + Nz(Me.TextBox3, 0)) / PopulatedFields
End Sub