I'm a little confused. We start with

  [ Header ][ ...variable padding... ][ Back Pointer ][ User Memory ]
                                       ^ always exactly sizeof(void*)
                                         bytes before User Memory,
                                         no matter how much padding
                                         came before it

and then it says we don't need to align the back pointer and we end up with

  [ Header ][ Back Pointer ][ Padding ][ User Memory ]

without a clear explanation of how we now get to the back pointer if it's behind the variable alignment.