how high....

so im new to indoor and i just got a 400w euro systems HPs i was wondering what the best height would be to hang it above my new babys ? their about 7 inchs tall and i have been using two 100w floro tubes and decided that i needed to get more light on these little guys, i have 9 of them; 4 sour diesel, 3 nothern lights and 2 super crystals, i took clones from the plants that i grew outside this season and used jumpstart from AV to help them root. the room size is 4ft by 4 ft and i put up mylar on the walls and put 6 inch pvc piping in one end of the room and another on the other side with fans on both so their is a pretty good constant air flow and the temp. has been around 81 degrees fahrenheit with the hps on, iv also installed a kind of double door so no light leaks out of my grow room. i plan on using Floranova grow and bloom with a little bit of fish emulsion and liquid bat guano are their any other good newts that i should use to get the most out of my setup ? im kinda a novice when it comes to newts. i have grown outdoor for a couple seasons but i am new to indoor
what advice can you give this newb....

thnks yall

Welcome to RIU!

I use the Flora Nova Bloom for the flowering phase and the mixture I have found to work well in a soil grow indoors is 1/2 tsp (it is super concentrated) to 2 gallons of water. I think the chart on the bottle is for outside growing with bigger plants. I also use a 400 watt HPS without a vented hood and have found that 18" is a good starting point between the light and young plants. As the plants get larger you can start to bring the lights a little closer to like 14". Good luck with your grow.