Choose the substitute that matches your goal: curry-paste swaps for aroma/body, and tom yum-style swaps for lemongrass tang.
Use the listed ratio as your starting point, then reduce or add paste based on your broth taste after warming.
If your substitute is watery, simmer it in a bit of oil for 1–2 minutes to thicken and bloom the aromatics.
Adjust balance: add coconut milk for creaminess, and add lime/vinegar 1–2 teaspoons at a time for brightness.
Taste again before serving; fine-tune heat with chili oil/chili flakes rather than adding more paste blindly.