how big is your space?what i would do with a 400 watt light is to section off an area about 2.5 by 3 feet.that is 7.5 square feet so you will get a little more than 50 watts per square foot which is perfect.my preferance is sea of green so i would pack about 20 clones in there and flower yeilding about 4 ounces give or take.but if you want taller plants i would put 5 or 6 and flower around 12 to 14 inches,they will finish between 2 and 3 feet and yeild a little more than sea of green but it will also take an extra 3 or 4 weeks to get the height you want.hope that i was helpful.i have been involved in cultivation since i was about 11 but it was all outdoor.i recently started indoor.cant wait to see how things went