Once I use up these 4x2" rockwool cubes I'm going to start using 6" cubes. Then I'll cut the slabs into smaller lengths so that I can sit the cubes into the tray flush with the slabs instead of sitting on top of them. I will have rows of three plants, with cubes on the ends and one in the middle, then 6" sections of slabs between the cubes that will hold the water rings.
This will shave off 4" of slab alone with the 4x2" cubes sitting on top, which will cause the rockwool to dry faster, making more frequent watering cycles possible. Plus it will dramatically improve the saturation of the root ball and cause a much more evenly distributed watering overall. Also it will drop the height of the plants a few inches, and make them more stable while being bent and twisted around during scrogging.