Daly's Bartenders' Encyclopedia by Tim Daly (1903, p.32)

1 pintchampagne — champagne (Pommery Sec)base
1 pintGuinness stout other
  1. Fill a large fancy glass half full with porter and the balance with champagne.
  2. Mix slowly with a spoon and serve.

This is a good mixed-wine-and-stout social service checkpoint from Daly's domestic/club bar world.

Daly emphasizes slow mixing and quality ingredients more than garnish or temperature theatrics.

See also: Champagne Cup, 'Alf and 'Alf

Source says for a party of four.