I'd probably add like 1cl lemon juice just to balance out the limoncello sweetness. I used Jarritos Grapefruit soda, which is on the tarter side of grapefruit sodas, and still found it bordering on cloying from the limoncello. Aside from that it's a refreshing, "European" version of Paloma.

Tried this for the first time this hot afternoon- very refreshing- Bombay Sapphire , Luxardo Limoncello and Capo grapefruit soda. Important to give it a brief stir as limoncello settles on the bottom when poured over ice. Not too sweet- just right.I'll tinker with bitters next time maybe a couple of drops of saline solution.