Remove strips of zest from 1 ruby grapefruit with a vegetable peeler, avoiding any white pith. Stir grapefruit zest and sugar together in a jar. Chill, covered, for 24 hours.

Squeeze juice from the grapefruit and measure out 1/2 cup juice. Pour 1/2 cup grapefruit juice, fresh lime juice, and water over the sugar mixture in the jar. Stir in kosher salt. Stir or shake mixture to dissolve sugar. Strain out zest; chill for 1 hour or cover and keep chilled for up to 3 days.

Combine salted grapefruit cordial, grapefruit juice, and lime juice in a punch bowl. Chill until ready to serve; pour in club soda before serving with citrus ice ring, if using.