Does Army full mess dress count as formal? If that is the case, I probably take the award for most formal events.

I would be wary about using a book from 1912 as a guide to modern court dress, especially as many class barriers have been broken down since that time. I would wear white tie, black tie, morning coat, full mess dress, or highland dress appropriately, but there are others who will show up at events wearing black tie or even suits these days. Cripes in the states, you even see blazers.

As one person pointed out, it is not unusual to see national costume at a formal ball in London, WDC, or Brussels. Traveling with the Army, I would be entertained by Arab or African wearing their native dress. I would look silly wearing full Arab dress as anything other than a costume.

Full highland dress is not really white tie. White tie is white tie and tails. While a full highland dress outfit can be worn to a white tie occasion, it should not be confused with white tie.