Identify your use: cocktail, digestif sip, or baking (where bitterness needs to be tempered).
Start with the listed ratio for your target: match bitterness first, then adjust for mint/herbal intensity.
If your substitute is sweeter than Fernet Branca (Averna/Jägermeister/Ramazzotti), use the reduced ratio or add a few drops of Angostura-style bitterness.
For cocktails, build the drink with ice and dilution, then taste after 30–60 seconds; for baking, mix thoroughly and bake before final judgment.
Taste and fine-tune: shorten sweetness with additional bitters, or soften aggressive herbal notes by slightly diluting with soda/water.