Hi madfreek,

I watched a grow guide on dvd the other day. The guy was using a flood and drain with clay pebbles just as you are. He flooded 3 times a day (while the lights were on) for 15 minutes each time (that was the minimum his timer would allow).
He had an overflow going bach to his resivour because in 15 minutes his tray would be full and it ensured no flooding.
Hope this helps.
Cheers