i believe justanotherbozo was correct in saying that 16 hours is not enough.

You could try putting it on 24/7 right away, and stick to that. I personally had my 400w hps on 20/4, and it worked out well for me. 18/6 would be good too, and probably the least amount of stress at this point. just dont switch the cycles too much, find the one that you think is best and keep it that way til flowering.