I want to tell you right off the bat that I am not trying to discourage you, but growing cannabis is not like growing tomatoes. It takes time, money, knowledge, dedication and love. But I am going to give it to you straight.
I am not sure what lights you have, but they are not 400W as fluoros like that are not manufactured at 400W....any where by anybody. I am guessing that the bulbs are T8s or T12s and 32 or 40W. While this will veg for a while it will not be adequate for later veg or flowering. Lighting needs are figured by lumens per sq ft. For vegging, you need a minimum of 3000 lumens per sq ft and for flowering you need 5000 lumens. The light for vegging needs to be in the 6500K range and the spectrum for flowering needs to be in the 2500K range.
You need ventilation also. You need to give your plants a continual supply of fresh air for proper photosynthesis. There needs to be an oscillating fan in your space to move the air around. Later you will need a good quality exhaust fan.
The box thing will not work. You absolutely have to have a separate place to flower. If you simply split the cardboard box, there is not way that you can make it light tight. In addition, they are not ready to flower, vegging and flowering plants need different spectrums, and that is not going to be enough light to flower. As a side note, you should be running your vegging lights 24/7. You probably have a month or so before they will be ready. A plant has to be ready to flower. Putting it in 12/12 will not cause it to flower unless it is ready. Your flowering space is going to have to be 100% dark 12 hours a day with absolutely no interruption to that dark period--i.e. you cannot stick your head in to see how they look. And 100% dark really means 100% dark. In addition you will need some kind of ventilation fan.
Miracle grow is not a good food for cannabis. It does not have the correct nutrients in the correct proportions. Just like vegging and flowering plants like different light spectrums, also, they need different foods. If a flowering plant is given the incorrect food, it will slow flowering substantially.
If this hobby was as simple as sticking some bag seeds under a fluorescent lamp everyone would be doing it. Unfortuantely it is not. Cannabis can be very finicky. And it takes a whole long time to grow. I just hate to seed someone spend 4-5 months on something that is not going to yield virtually anything. You may decide that it will be in your best interest to wait and just plant everything outdoors when the last danger of frost has passed. However, that too is going to take good soil and the right nutrients.
Like I said, not trying to discourage you, but just want to make sure you have a better grasp about what growing indoors entails. Like I said, I would hate to see you 4 months down the road with nothing to show for it.