How much ballast should you use in a bottle rocket?

The amount of ballast used in a bottle rocket typically depends on the size of the bottle and the desired flight characteristics. A good starting point is to use around 10-20% of the bottle's total weight in ballast. It's important to experiment, as too much ballast can hinder flight, while too little may result in instability. Adjust the ballast based on test flights to achieve optimal performance.