The more lumens put out by the light, the fatter your buds will be in the end. Your 400W HPS bulb probably puts out somewhere near 45,000 lumens. Depending on how much space you have, you can determine roughly how much you'll be receiving per square foot. That's a good starting point. If you feel that the light isn't strong enough, you can always add more...or upgrade to a larger system. Besides heat restrictions, there is no overkill when it comes to light/lumens during flowering. At least, I've never heard anyone complain of having too much light before. The more you have, and are able to sustain, the fatter your final yield will most likely be. Just remember to match how much you spend on lighting with about how much you spend on ventilation. Do that, and upgrade to your hearts content! :jointsmile:

Here's a website I came across recently. I haven't had the time to play around with it, but the yield calculator looks pretty interesting: Indoor YOR