Question help how to charge monthly fee of multiple customer

[trialer]

how can i add the newbal to datagridview with databound already?

 Private Sub cal()
        i1 = PrevBal
        i2 = MF
        calc = i1 + i2
        NewBal = calc.ToString
    End Sub

    Private Sub Button2_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button2.Click
        Call cal()
        With DataGridView1
            .Rows.Add()
            .Rows.Item(.Rows.Count - 1).Cells("NewBal").Value = NewBal
        End With
    End Sub

help... thanks..