Treat it like you would any potting soil. Pack the root system (rockwool cube) in snugly, but don't berry the stem of the plant. In a water culture system you'll want the the bottom of the growing media to get moisture. But just barely, and allow it to wick up the moisture through the growing media. If using two different growing medias (like rockwool and hydroton) witch have two completely different property's, you need to take both into account. Rockwool will become saturated much quicker than grow rocks. You'll want the grow rocks to be able to be moist, but not so much that the rock wool is saturated. if the rockwool stays saturated, you'll likely wind up with stem rot. I always suggest building your system (any system) so the water level can be adjusted. Then if the water level is getting to high, just lower it to a good point, and Vice-versa.