Ideally the water level should be right around the bottom of the net pot. And inchnor two below won't hurt a thing. The danger in DWC is getting the water level too high not too low. Water levels that are too high in a DWC setup, particularly in places with a lot of salt in the water, can cause basal stem disorder or elephant's foot. If this happens your plants will die before your eyes. Not cool. Never let the water get anywhere near the area where the stem meets the medium. I know this sounds stupid but you'd be surprised how many people do it and lose their plants in a flash. Also just as a side note; If you're not already, be very conscious of your water temps. Absolutely crucial in DWC.