Dutch Passion did a study on how to influence sex in marijuana plants. Take a look at it. Sex seems to be determined around week three to week four, and there are several things they advise doing and NOT doing during the 3rd to 4th week, or about the time your plant has three sets of true leaves.
Stress is a major factor in determining sex. The more you stress young plants, the more males you get. Stress is caused by transplanting, dramatic change in or interruption of light cycle, cold, heat, underwatering, overwatering, nute burn, etc. Best ot keep your plants as comfy and healthy as possible for the first 4 weeks. A soil nutrient content that is highest in N and lowest in K is also beneficial. Maybe wait four weeks to put them in 12/12. They also said to use high spectrum blue light (6500k) to increase your female chances.
I've used a lot of these tips and typically get 80% females. At three weeks I give them a 1/4 strenght shot of Growbig 6-4-4, and I use 6500k cfls for young seedlings. Hope this helps.