The other good thing about using a rock cube for your dwc.. fits right into the basket.
i'd want the roots to be fairly close to hitting the res, i would assume.. but you need someoen with more experience. if you have a drip system up top, you can do it immediately and not have to water at all.. if roots aren't hitting res, you have to top feed until they do to provide enouhg for plant -- not just dip toes on.