That 400 watt light can reach half a pound... that's only about .6g/watt. However, a pound would require some damn good growing on your part, it's nearly impossible to reach 1.2g/watt especially with only 1 plant.
You will have to supercrop/LST/Scrog it so that the top is a wide surface... and try to make it fit into about 2.5x2.5ft. If you try 3x3ft it will barely grow on the outside and 2x2ft is a little cramped.
Keep the 400watt light less than a foot above the top of the plant (as close as possible without the reflector blocking light to the outsides... but not so close to burn). As soon as you see even a small bit of yellow on the tips of leaves, cut back on the nutrients a little. Nutrient burn will be the biggest way to screw up, even a slight nutrient burn will cause a decrease in yield.
If you can get a Son Agro HPS(technically 430 watts) or something else with an added blue spectrum, it'll help.
For the hell of it, you might want to try a light cycle of 16 hours on, 12 hours off (every day, turn the lights off 4 hours later). It'll take another 2 weeks to finish flowering, but it may give more growth in the buds.
