Identify your use: browning/roasting vs. baking/structure, since fat behavior changes results.
Start with the closest ratio from the list (usually 1:1, or ¾ oil : 1 butter for blends).
If the substitute is softer or liquid (like olive oil), reduce the cook time slightly and watch browning closely.
For baked goods, choose a solid fat (lard, ghee, shortening) to maintain flaky or tender crumb; chill if your recipe allows.
Taste and adjust: if flavor is missing, add a small boost of salt or a small amount of butter flavoring next time rather than changing ratios mid-bake.