Specifics of botanical cell?

Hi everyone!
We are building a small botanical cell and I was just wondering about the water level and fill material.  It is a little unclear in the earthship book I have.  

Should the water level (the level of the overflow) be below the sand layer or should it be in the dirt level?  In the book it looks like the level of water is in the lower part of the dirt layer but it seems that could cause root rot?  Any help would be much appreciated!

Thanks,
Kadin

I haven't come across these 'botanical cells' (they sound a bit like wicking beds, but with extra function) I did find this site:

All of the grey water planters will contain 18″-3′ of rock below a layer of dirt. The overflows are located such that the water level cannot rise above the top of the rocks … the water is restricted to the rock layer. The plants will grow in the dirt layer with their roots extending down into the rock layer. Hopefully none of them will get washed away!

They also go into a bit of detail on the levels of the planters, so that the water flows without pumping from the sinks and appliances, but it sounds like they recirculate it as well?

I found this image on a couple of sites:

source

But the fine detail is not legible.

In a wicking bed I believe the soil is above the level of the water which is drawn up by capillary action, so I would put the overflow somewhere in the sand level. It is probably worth making it something that is adjustable however, as the optimum level may depend on the plants and your temperature etc.