VBA Calcul direct.

Dim x
x = (valeur_tableau_source * Range("G9").Value) / Range("G5").Value

Private Sub CommandButton13_Click()
Dim Variable1 As String
Dim Variable2 As String
Dim Variable3 As String
Dim Variable4 As String
Dim Variable5 As String
Dim Variable6 As String
Dim Variable7 As String
Dim Variable8 As String
Dim x As String
Dim i As Integer
Dim j As Integer
Dim h As Integer

        i = 9
        For i = 9 To 846 Step 27
          If Cells(i, 2) = "753220 - PARIS KELLER ACP" Then
                For j = 5 To 26 Step 1
                    
                       If Cells(i + 1, j) = "Janvier" Then
                               With ThisWorkbook.Sheets("Détail ACP")
                     
                                    Variable1 = .Cells(i + 8, j).Value
                                    Variable2 = .Cells(i + 12, j).Text
                                    Variable3 = .Cells(i + 11, j).Text
                                    Variable4 = .Cells(i + 3, j).Text
                                    Variable5 = .Cells(i + 14, j).Text
                                    Variable6 = .Cells(i + 15, j).Text
                                    Variable7 = .Cells(i + 5, j).Text
                                     Variable8 = .Cells(i + 6, j).Text
                                End With
                        Set Dest = Workbooks.Open(ActiveWorkbook.Path & "\TBM ACP.xls")
                         With Dest.Sheets("Paris Keller")
                              .Cells(9, 7).Value = Variable1
                              
                   x = (Variable2 * .Cells(9, 7).Value) / .Cells(5, 7).Value
                   .Cells(13, 7).Value = x
                              .Cells(30, 7).Value = Variable3
                              .Cells(40, 7).Value = Variable4
                              .Cells(42, 7).Value = Variable5
                              .Cells(43, 7).Value = Variable6
                              .Cells(45, 7).Value = Variable7
                              .Cells(47, 7).Value = Variable8
                         End With
                 Exit For
          End If
        Next
      Exit For
    End If
  Next
  
 
End Sub

x = (Variable2 * .Cells(9, 7).text) / .Cells(5, 7).text
                   .Cells(13, 7).Value = x

Private Sub CommandButton1_Click()
Dim Variable2 As Single, Dest As Workbook, x As Single
With ThisWorkbook.Sheets("Feuil1")
    Variable2 = .Cells(1, 1).Value
End With
Set Dest = Workbooks.Open(ActiveWorkbook.Path & "\destination.xls")
With Dest.Sheets("Feuil1")
    x = (Variable2 * .Cells(1, 2).Value) / .Cells(2, 2).Value
    .Cells(3, 2).Value = x
End With
End Sub