From what I've heard, 20/4 is the best light schedule you can go with for an auto strain.. don't take my word for it though, I'm about to grow autos for the first time too
let your lights run for 24/0... The less light you use the less weed you get. I have 2 RRF's under 1000w right now and the calyx's are almost as big as my pinky finger nail.