They sell float valves made to take a gravity feed or pressure feed. I highly recommend gravity feed because it means that a failure dumps a bucket of water... a pressure feed means a failure goes until someone stops it. In fact, the local shop, as far as I can tell, currently only carries the gravity feed type. They have them with adjustable float arms now too, so you can adjust the level.
I put one in my res bucket and connect all the others to it on the bottom. Then I pump out of the res bucket to the top of the plant buckets. This lowers the level so the end level of the system is actually slightly above the normal fill line for the float. It is possible to go the other way or use a slower continuous flow, but for simplicity this works fine.
Then just mark a bucket off in 1/2 gallons, and connect it to the float input. Set it on top of the resevoir (I like putting a valve on this so I can shut it off and disconnect it). Then just refill it to the top line and when you come back later, you know exactly how much it has used, and the level on the plants is consistent.
I think I paid $30 for the fancy adjustable float, but have used much cheaper ones in the past. If you want to get really creative, replacement parts for toilet tanks could easily be used.