you would leave it on 18/6 for several weeks usually 4-6 weeks and then switch to 12/12.
if you are growing in a cabinet you may need to switch to 12/12 sooner depending on how much room you have to work with as most plants can double in size when you switch to 12/12 and you do not want to overgrow your space.
you can probably get the 400 within 7-10 inches of the tops of the plants and you may want to lower it in about a week.