Identify what the port wine is doing: sweetness (desserts), tang/brightness (sauces), or body after reduction.
Pick a substitute with the closest sweetness level; start with the ratio listed in the reference table.
If your substitute is drier than port, add a small sugar correction (about 1–2 teaspoon per ½ cup) before reducing.
If your substitute is more acidic (balsamic or vinegar-based), simmer/reduce until it coats a spoon, then taste and fine-tune.
For baking, mix the substitute into liquids first so it disperses evenly; bake as directed and adjust sweetness in glaze/icing only if needed.