the way I have mine set up is tray with a layer of hydrotron, then 4" cubes and then more hydrotron about 1/4 to 1/3 of the way up the cube. so simple terms 1/3 to 1/4 of cube is "buried" in Hydrotron. the flood level goes about 3/4 the height of the cube. from day 1 I never water the cubes from the top I always hand dip them that way I believe I might be training the roots to go down looking for what they need. If the top section of block gets a lil dry I dont worry much since roots on bottom anyways but as we all know the whole RW cube is going to be "wet" to a certain point no matter how you water it I just try to concentrate on the lower half or so of block.