I am curious about the spreader material too, and the NFT/rw hybrid system you are running.
Can you post pics that show the system assembled, and how it all sets together? What is the spreader mat for in your system? How does it fit in?

On one hand a RW slab holds plenty of moisture and can be fed with a slow drip, or by ebb and flow bery effectively. On the other hand, you are running towards NFT. If that slab is sitting IN the NFT channel, it will be too wet, all the time.

I've seen leach trays that seem well suited to holding a RW slab ABOVE the NFT channel so that you can start out doing drip or flood until the roots are long enough to go into the channel, and then stop dripping and go true NFT. They are shown actually in the thread 'stinkys adventures in noob hydro' in 'grow logs' under 'basic growing', which is a hybrid NFT system, and was also finicky until the root system was established.

The trick with what I think you are trying to do is finding the balance between overwatering the slab and underwatering the protruding roots.