How many plants did you have under that 400w, and how far away was it? When I had my 400, it was air cooled, and I kept the glass 4-5 inchs from the tops of the plants. The smaller the light the closer it needs to be, and the less plants you belong running. Also how tall were the plant(s), a 400W light will not have much canopy penetration. If the plants were more then a couple feet tall, then the lower branches and bud sites would not be getting much usable light.
I agree with Pet about not needing to boost to much during flower. I run the same nute mix from the day they go into flower till the day I harvest them(for the most part). I also run a perpetual garden so I have plants that are young, and fully mature in the same flood tray all the time. I have always followed the KISS method, and feel its much more important to give the plants what they need, rather then any EXTRA other stuff, or overly high amounts of things.