You can get at least down to 88% in the common case, since there's an unused bit in almost all characters (7 bit ASCII in 8 bit bytes). To actually realise this

You can get at least down to 88% in the common case, since there's an unused bit in almost all characters (7 bit ASCII in 8 bit bytes). To actually realise this in practice, one would have to use an encoding that works with the Alphanumeric or Numeric QR modes. (And yeah, dictionary compression would be great on this: lots of repeated text like "Pty Ltd" and "Cafe".)