For DWC, I put the nute level at mid point of the net pot, especially if the plant is a small one so I dont run the risk of it drying out. It really dosent matter, Ive even filled it up till it was over flowing. Im pretty lax on my 2.5 gal buckets and I get really good results and Ive had those plants going for two years. On the other hand when I put out my 10 gal tub with 16 sites, I have to keep a closer eye on this one as its outside and it runs the risk of losing nutes quicker, but same thing, I'd rather have it full to the top than almost empty. In reality, its said that the nute level should be right under the net pots as the bubbles burst against it and keep the medium moist, but its up to you. The real key is to keep a very humid oxygen rich environment for the roots.