Bonjour,
Un exemple à adapter.
Cdlt.
Public Sub Calcul()
Dim ACell As Range, Cell As Range
Set ACell = ActiveCell
Set Cell = Application.InputBox(prompt:="Sélectionner la cellule.", _
Title:="Calcul de différence", Type:=8)
If Not Cell Is Nothing Then _
ACell = ACell.Offset(, -1).Value - Cell.Value
Set Cell = Nothing: Set ACell = Nothing
End Sub