You have to figure with a 400watt light, even at a gram per watt, which you will not get, you are still short 2oz from a pound.
If you are using a 400watt light, its your first grow, expect a couple of oz's and be happy with that. After you get some experience, a larger light, better equipment, etc, you will be able to pull a pound. But don't expect much more than 1/4 of a gram per watt of light in your first grow or two.