60 watts of light is needed per square feet. In a 36"x20" space, 400 watts is going to work out just fine. Distance to the plant should be measured with the back of your hand. Hold the back of your hand flush with the tops of your plants & see if the light burns your hand. If you can hold your hand in place for a minute or so without it burning then you're fine but if it hurts you, you will need to adjust the light accordingly. I peeped your 1st grow journal btw, awesome grow for your 1st time.