Decide if you need grapefruit’s bitterness (closer match) or mostly its tart lift (safe for most cakes, muffins, and glazes).
Use the listed ratio as your starting point, then adjust by adding lemon juice (more tang) or a small pinch of sugar (less sharp).
If the substitute has solids or strong pulp, strain before mixing to keep batters and glazes smooth.
For baked goods with rising/structure needs, avoid overly acidic substitutes concentrated with vinegar; dilute and taste the batter if possible.
After baking, evaluate the finished flavor; next time, fine-tune by tiny increments of lemon or sugar rather than big changes.
