Hey parkingjoe, using RW slabs in an ebb and flow defeats the high oxygen replacement goals of using the ebb and flow system.
I've used a Hydroton/River-rock 50-50 mix for years and had excellent results with it.
The goal of using ebb and flow is to maximize the oxygen to the roots. Using Hydroton/rock, each time the system drains, it pulls fresh oxygen all the way to the very bottom of the entire root mass and the roots feed from the residual moisture that is retained by the porous hydroton and that which clings to the river rock. This makes the water/nutrient/oxygen supply max out.
However, RW slabs retain much too much water after draining. The mass of roots within the rockwool won't benefit from the fill/drain cycle nearly as much as they could if the Hydroton/rock mix was used.
RW is good as a replacement for dirt grows where a hybrid system using hydro nutes is preferred. Massive feeders like Tomato plants, which use as much as a gallon of water per/day per/plant are excellent choices for using RW, but only as a dirt replacement where the RW is drenched each feeding in a container grow. The slabs would be spaced to provide just enough room per/plant for root growth and the slabs would dry out almost entirely each day by afternoon when they are filled again with only the amount that starts a little run-off.
If you'd like to build an ebb and flow that is exactly what will fit in your grow area and is designed specifically for MJ, let me know. I'd be glad to help you construct one that will fit your needs.
As for the feeding schedule; After years of working on this, I've come to the conclusion that with full grown plants in a high vegetative state of growth and using the Hydroton/rock mix, a one hour on/one hour off cycle works best to maximize both oxygen and nutrients/water to the plants.
The hour on/hour off cycle provides the most oxygen to the roots while also providing maximum water uptake and nutrient delivery to the plant.
The goal is to make the plants environment that which prevents the plant from having to do ANY work to gain either max oxygen, water or nutes.
The root system stays relatively small and shows that the plant is gaining what it needs without massive root growth to compensate for inadequacies in delivery.
I have slightly more than 25 years of experience using ebb and flow, drip and NFT systems and have designed many hundreds of units to custom preferences of the owners. I've also been able to monitor the plant growth in most of these systems I've built and have tweaked my own system until it has nothing left to improve.
Please let me know if I can be of help to you.
Stoney.