Dip the rim of a highball glass in a bowl of water, shake off the excess, then sprinkle a spoonful of salt onto a plate or saucer and dip the glass into that to coat the rim.
Fill a cocktail shaker with ice, then add all the remaining ingredients minus the soda water. Shake until the outside of the shaker feels cold, then strain into your prepared glass. Add a few fresh ice cubes and top with soda water to serve. Garnish with the wedge of grapefruit.