The height of the plant really has nothing at all to do with whether it is ready to flower or not. What you want to look for is alternating nodes and preflowers. It didn't look to me like you have alternating nodes yet. This is when the branches do not grow out straight across from one another, but rather alternate up the stem. You have tons of room left in that tent--the fan can always be moved outside the tent or raised to the upper part of the tent. If you flower now, your yield is going to be very small. If you can get a little bushiness to them before you put them into 12/12 your yield would be vastly improved.
A 400W ballast running at full power puts out 54,000 lumens. If you are running at 50% power, you are putting out 50% less lumens. So that would be 27,000 lumens, which is still okay for a tent that size.
I can see your fan there, can you tell us how you have the ventilation set up and what your temps are? Most people either mount the fan high or have it located outside the tent so that it does not take up valuable floor space.
What are you feeding your plants?
Do you know the strain or are these bagseed?