Lumens/footcandle question

I have a 4'x4'x6' growing space. I'm using a 400w hps (the link will be more helpful, just ignore the audio). I'm curious as to how many lumens does a flowering plant need and whats the ranges i.e. 10,000-30,000? maybe 50,000?
http://youtu.be/TX3agc9aia4