VBA Code & Formula Interference

I don't really understand what you want but try this

Private Sub Worksheet_Change(ByVal Target As Range)
    If Intersect(Target, Range("P:P")) Is Nothing Then Exit Sub
    If Target.Cells.Count > 1 Then Exit Sub

    'Turn off events to keep out of loops
    Application.EnableEvents = False

    If Target.Value < Target.Offset(0, 1).Value * 2 Then
        Target.Offset(0, 1).ClearContents
        Target.Offset(0, 1).Select
        MsgBox "Re-enter a valid value in " & Target.Offset(0, 1).Address(False, False)
    Else
        Target.Offset(0, 1).ClearContents
        Target.Offset(0, 1).Select
    End If

    Application.EnableEvents = True
End Sub