The nutrient solution should be kept at a level of 1" above the bottom of the net basket when a new plant/clone has been introduced to the bubbler.
After a good root system has been established, the level can drop down in the bucket and will create a nice aeroponic type atmosphere for the roots to thrive in. Make sure to keep an eye on your TDS however,a s a significant drop in the reservoir level can mean a significant increase in your TDS level. With frequent checks using your TDS meter, you should have no problems. When TDS levels rise in the bucket, you can add plain pH adjusted water to top off and adjust TDS to the correct levels.