Sub Worksheet_Activate()
Dim L%, Plage As Range: L = 2
[C:C].ClearContents: [C1] = "Reste"
With Sheets("Stock")
Set Plage = .Range("A2:B" & .Range("A65500").End(xlUp).Row)
While Cells(L, "A") <> ""
On Error Resume Next
Cells(L, "C") = Application.VLookup(Cells(L, "A"), Plage, 2, 0) - Cells(L, "B")
L = L + 1
Wend
End With
End Sub