Sorry, but the questions keep popping into my head - if you water from the top, (using the yummy worm-casting enriched water) do all the pockets get moist, all the way down? Or will you give it a shower with the hose once in a while? Or have to water the lower tier directly sometimes?
A better way to state this question would be: How much water does it take to moisten the whole tower all the way to the bottom tier? Could you let it dry a bit, and then measure this with a watering can or something, Dave?
I guess it's also going to vary depending upon how much the upper tier plants "drink" on the way by.
One more - how much potting soil did it take to fill it up?