Ebb and flow nutrients

I have a 5x10 tent. 3 600 digital ballasts. 12 site ebb and flow with 55gal. reservoir. 7gal. control unit. Ppm pen (waiting for arrival) ph pen (waiting for arrival) ph up/down. I have an 8" exhaust fan with filter exhausting out. 6" fan with filter bringing clean air in. 3 part gh nutrients gallon size jugs. Clay pebble medium. I've never used the flood and drain system. My biggest concern at the moment is how much water should I fill my 55gal. reservoir. Also how about measuring nutrients for this reservoir. Advice on flush cycles? Any good qualified individuals input would be greatly appreciated. This post should be screaming nub all over it lol.