Ratios of gin:port range from 2:1 (Craddock) to 4:1 (Difford) so I split the difference and went with the 3:1 specified by Kappeler and Embury, omitting sugar syrup since I have Hayman's Old Tom. Difford says to use tawny, which I did (10-year-old Quinta Das Carvalhas), but the earlier sources just say "port" and I think it would be good with ruby, too. As for pouring the port *through* the chilled gin & bitters to settle in the bottom of the glass, this appears to be an acquired skill as I managed to get enough port all over the countertop to make a mess. A small measure with a pour spout would help. It does produce a pretty drink, however. An extremely effective use of just four ingredients including bitters and expressed orange zest; everything contributes.