How do I go about ensuring the concentration of Surfex-HD is correct for my pressure washer?

Fill up the detergent bottle or reservoir with exactly one litre of water. Take a large bucket or container and discharge the lance into this until the detergent bottle is empty. Measure the total amount of water collected, and multiply this number by 0.04. Fill the detergent bottle or reservoir with the calculated amount, and make it up to one litre with water. This method will create the correct concentration of Surfex-HD discharging from the lance.