how to make a calculated field save into sql table?

Hi,
I have a form that has input,used and output,the output is a calculated field that i don't show in the form but i have it in my sql table,the user inserts output and used and i want to be able to calculate the output and save itdirectly to my sql table
i thought of doing a hidden field that reads from the input and used and then calculate it as follows:
output=input - used

but i don't know how to read from the fields ,below is my insert code:
*************************************************************************
Try
  Dim MySQL As String = "Insert into totals(input,used) values(@txttext1,@texttest2)"

 Dim myConn As SqlConnection = New SqlConnection("data source=dcmp;uid=sa;pwd=;database=bookshop")
            Dim Cmd As New SqlCommand(MySQL, myConn)
            Cmd.Parameters.Add(New SqlParameter("@txttext1", TextBox1.Text))
            Cmd.Parameters.Add(New SqlParameter("@texttest2", TextBox2.Text))


End Try
        BindData()
        'myConn.Close()

    End Sub

    Sub BindData()

        'Dim MySQL as string = "Select * from totals where [bookid] = @id"
        Dim myConn As SqlConnection = New SqlConnection("data source=dcmp;uid=sa;pwd=;database=bookshop")
        Dim MySQL As String
        MySQL = "Select * from totals order by dbookid "
        Dim Cmd1 As New SqlCommand(MySQL, myConn)
        'cmd1.Parameters.Add(New SqlParameter("@id", tmstm.text))

        Dim ds As DataSet = New DataSet
        Dim Cmd As New SqlDataAdapter(MySQL, myConn)
        Cmd.Fill(ds, "totals1")
        mydatagrid.DataSource = ds.Tables("totals1").DefaultView
        mydatagrid.DataBind()
***********************************************************************************

is it the right way to use hidden textbox or there is better way?