I only grow autos...and I personally have never grown one on a 12/12. I have tried several different cycles....but the lowest I ever went with the light on was 16.....so it was a 16/8......I now use 24/0 for my autos.
I only ask as i just put the blue dream auto in my flower tent for a bit of early smoko and was wondering if anyone else does the same as its a first for autos for me