What is meant by ‘achieving X% of runoff volume’?

This means to continue watering until an amount of Clearex solution equivalent to 80%-90% of the volume of the container being used occurs. For example, if you are growing in one-gallon pots, you would flush with the Clearex solution until you achieve a little under a gallon of runoff. Prior to harvest Clearex only needs a single, thorough, application. For the remaining week before harvest, use plain water without any nutrients or additives.