Hello fintuckyfarms,
Don't worry about the top plants using the
nutrients up before the lower plants. If that were an issue or even possible, recirculating systems would not work. In other words if the nutrients were used up after the first pass through the system, there would be no point in circulating it through the system again because by the time you pumped it back up to the top plants it would be all used up. At the same time you would need to worry about the very first plant in a single row using up all the nutrients first as well, but it just dosen't happen that way.
The issue you will need to deal with in a stacked flood and drain system is pluming. That is getting each level to flood to the desired level, then drain back into the reservoir evenly. But it can be worked out one way or another.