Choose the substitute that matches your dish: closest match (brandy) or flavor-workarounds (sherry, whiskey, wine).
Use the listed ratio, then start with a slightly smaller amount if your recipe already tastes sweet or boozy.
Adjust balance: add 1–2 teaspoon water for harshness, a tiny pinch sugar for flatness, or a small splash vinegar/lemon for lift.
Add it at the same stage you’d add cognac (for reductions, add early to cook off alcohol; for finishing, use later).
Taste the sauce after simmering a few minutes—sweetness and oak soften as it reduces.