Option Explicit

Private Sub Worksheet_Change(ByVal Target As Range)
      If Intersect(Target, Range("B1:B3")).Address = Target.Address Then
          Dim c As Range
          For Each c In Target
              Dim sProc As String
              sProc = Format$(c.Value / GetPlan(c.Offset(, -1)), "0.0%")
              AddCommentIn c, "план выполнен на:" & Chr(10) & sProc
          Next c
      End If
End Sub

Private Function GetPlan(r As Range) As Long
      Dim sTemp As String
      sTemp = r.Comment.Text
      sTemp = Replace(sTemp, "План ", "")
      sTemp = Replace(sTemp, "тыс", "")
        
      GetPlan = CLng(sTemp) * 1000
End Function

Private Sub AddCommentIn(r As Range, sText As String)
      On Error Resume Next
      r.Comment.Delete
      r.AddComment sText
End Sub