Revisited after a couple years in an effort to finish off my bottle of Canton, and I still think this is an amazing drink. I do prefer it with a bit less lime however, and the last couple of times I made it I added a little orgeat since it plays so well with the Fernet Branca and the ginger. I amend my previous comments about the ginger beer being optional- it's a little syrupy without. However, the drink is on the sweet side and the ginger beer only adds to that. I've heard that King's ginger liqueur has more ginger flavor than the Canton that I finally finished, so I wonder if that with regular soda water would give enough ginger kick without getting too sweet.
Never having tried it with Canton, I just made this with King's Ginger, 3/4 oz of lime, and soda instead of ginger beer. I thought the flavor balance was good, not too sweet, but the ginger was more subtle than big-kick.