With all of your waterfarms hooked together and into the controller, just fill up the system and add the nutes appropriate to the total amount of water in the system. In your case it is not really a controller, it is an external reservior. But that is just as good. Now you add water, test pH and EC, and flush all from one source.

When you fill your system for the first time mark with a laundry pen inside the controller the water level. Then each day add fresh water to fill it back to the line. Or if you got the float valve, just attach that to a 1/4" line of fresh water.