Made this with. 1 1/2 cl coffee liqueur due to a comment about it being too subtle. It wasn't subtle at all! (I used Galliano ristretto). The coffee arrives on the palate late and prominently along with the bitterness. I think Diffords measuments would strike the best balance. Nice drink, although a bit sweet and doesn't beat a regular negroni.
I tried making this with the listed specs and found it way to sweet and cloying (I used Villa Massa and a brut cava). Made another one with 3-2-1 parts Cava, Limoncello and soda water, which was delicious.