Wayne Collins, this drink's creator, originally used equal parts as is usual for a classic Negroni and after trying various other formulations, I've found...
I'm about to add a variant of this recipe that was available at a now defunct restaurant called Prospect, on Fulton St in Fort Greene, Brooklyn: called the Polka Dot (cool name!), it uses blanc vermouth instead of lillet and (convenient) grapefruit bitters instead of the grapefruit garnish, and a garnish of luxardo cherry