Water depth in RDWC system

First off, I have a 21 gallon reservoir with 4 buckets attached. Yet Im only using two at the moment. Now does it really matter how far the water/nutrient solution in the bottom of the bucket is from the netting in the buckets which is holding my 2 inches of hydroton and rockwool cube which are getting watered via a drip ring? I,ve heard 2 inches from bottom of net after roots start stretching, but shouldnt the water levels just stay the same at all times. Im using aquashield and clearex