The Ebb'N'Flow system is based around your water pump. This means that as long as the pump is supplied power, it will continuously pump water at a constant rate until it is turned off. The pump rests at the bottom of your reservoir. The water your pump supplies goes UP a tube to your "flood" tray; where, you guessed it, the water floods the tray. To set the water level, where your makeshift flood reaches its maximum, set your drainage hose at exactly that level.
Most ebb-n-flow systems come with little pipes you can connect to easily change your water level as your plants get bigger. If you're building a DIY ebb-n-flow (which is pretty easy), you may want to consider buying the inexpensive little drainage pipes at your local hydro store.
Your drainage pipe/hose then releases all the water that goes above your water level back into your reservoir where it can be mixed and pumped back into your flood tray.
I feel that I made this a little more complicated than I should have, but I hope it helps. Be sure to water at most 3 times a day for rokwool (depending on how much water your plants will use before you water again), and at least 3 times a day for hydroton, unless your humidity levels are extremely high.
peace out,
cogogrow
