Identify the flavor job: anise/licorice lift (liqueur-like) vs. bitter herbal backbone; choose the substitute that matches your goal first.
Swap using the listed ratio (aim for the closest match), then taste once your drink/dessert base is combined.
If the substitute is sweeter, add a splash of citrus or a tiny pinch of salt; if it’s too bitter, increase sweetness slightly.
For very intense bitters (like Fernet), start lower and scale up gradually so you don’t overpower the recipe.
Keep the finish consistent: let the mixture sit 5 minutes (or chill briefly for cocktails) so flavors meld before final adjustment.