You will never reach a half O/plant if you start seedlings off at 12/12, shit you'll probably only get a joint, seriously. You must first vegetate the plant which is simple, Just switch the lighting cycle to anywhere between 14/10-24/0, the increase in light will cause the plant to grow (Just think of it as spring/summer for your plant) then when your plant reaches a little more than half the size you want it to be, switch over to 12/12 (think of it as autumn for your plant)Thats when it will start budding and you can determine sex. Some folks will switch the light from veg to flower to determine sex, then switch back to veg so that they can get rid of the males early on. Any more questions feel free to ask, I have many answers.
Peace