Datagridview Cells value calculation

I have 18 column i can insert database easily also i can perform Cell 17 calculation with CellEndEdit, but i wanna perform calculation with btnAdd click event.

NOTE: S.No. column auto generated

Public Class Form1

    Private Sub Form1_Load(sender As System.Object, e As System.EventArgs) Handles MyBase.Load
        Me.Tbl_SaleTableAdapter.Fill(Me.Ds_Sale.tbl_Sale)
    End Sub

    Private Sub dgvSale_CellEndEdit(sender As Object, e As System.Windows.Forms.DataGridViewCellEventArgs) Handles dgvSale.CellEndEdit
        If Me.dgvSale.Rows.Count > 1 Then
            With Me.dgvSale.Rows(e.RowIndex)
                .Cells(16).Value = .Cells(7).Value * .Cells(13).Value + .Cells(14).Value + .Cells(15).Value
            End With
        End If
    End Sub

    Private Sub Button1_Click(sender As System.Object, e As System.EventArgs) Handles btnAdd.Click
        Dim conn As New SqlServerCe.SqlCeConnection("Data Source=|DataDirectory|\Database.sdf")
        Dim cmd As New SqlServerCe.SqlCeCommand
        Try
            cmd.CommandText = "INSERT INTO tbl_Sale (Month, CoName, HireDate, LPONo, MRANo, Location, HireRate, ContactPerson, SupplierName, GivenRate, InvoiceNo, InvoiceMonth, Workingdays, Salik, MiscCharges, Comments) VALUES (@Month, @CoName, @HireDate, @LPONo, @MRANo, @Location, @HireRate, @ContactPerson, @SupplierName, @GivenRate, @InvoiceNo, @InvoiceMonth, @WorkingDays, @Salik, @MiscCharges, @Comments)"
            cmd.CommandType = CommandType.Text
            cmd.Connection = conn
            With cmd.Parameters
                .AddWithValue("@Month", txtMonth.Text).DbType = DbType.DateTime
                .AddWithValue("@CoName", txtCoName.Text)
                .AddWithValue("@HireDate", txtHireDate.Text).DbType = DbType.DateTime
                .AddWithValue("@LPONo", txtLPONo.Text)
                .AddWithValue("@MRANo", txtMRA.Text)
                .AddWithValue("@Location", txtLocation.Text)
                .AddWithValue("@HireRate", Val(txtHireRate.Text))
                .AddWithValue("@ContactPerson", txtContactPerson.Text)
                .AddWithValue("@Suppliername", txtSupplierName.Text)
                .AddWithValue("@GivenRate", Val(txtGivenRate.Text))
                .AddWithValue("@InvoiceNo", txtInvoiceNo.Text)
                .AddWithValue("@InvoiceMonth", txtInvoiceMonth.Text).DbType = DbType.DateTime
                .AddWithValue("@WorkingDays", txtWorkingDays.Text)
                .AddWithValue("@Salik", txtSalik.Text)
                .AddWithValue("@MiscCharges", txtMiscCharges.Text)
                .AddWithValue("@Comments", txtComments.Text)
            End With

            conn.Open()
            cmd.ExecuteNonQuery()
            Me.Tbl_SaleTableAdapter.Fill(Me.Ds_Sale.tbl_Sale)

        Catch ex As Exception
            MsgBox(ex.Message)
        Finally
            conn.Close()
            cmd.Parameters.Clear()
        End Try
    End Sub
 
End Class