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...
There’s a Classic Martinez recipe in the Cocktail Codex: 1.5 oz gin, 1.5 oz sweet vermouth, 1/4 oz Luxardo. I like these ratios substituting in blanc vermouth and Suze. I appreciate the bitterness but its a little heavy for me, one to one.