A half-ounce per plant is definitely doable with a 400W light. There are other factors, though. You need a good basic NPK, and some bud boosters like Bud Candy, Massive, and maybe Gravity. Be very careful with Gravity if you use it though, as it can burn plants easily. These bud boosters make your flowers bigger, denser, and harder. The added boost in sugars and cal/mag I have found increases my harvest weights.
The other crucial factor in this is your light. If you grow your plants as they are now, they'll be big single colas. They can yield a half-ounce bud each, but they need to have to have the light about 16-12" above the canopy. If your plants reach a height of about 18-24" when they finish, you can expect a half-ounce per plant. If you are only shooting for a half ounce with a 400W you can raise your expectations a little. If you have 6 plants, at a half ounce each, you could have a nice little 3 ounce harvest.
The other crucial factor in this is your light. If you grow your plants as they are now, they'll be big single colas. They can yield a half-ounce bud each, but they need to have to have the light about 16-12" above the canopy. If your plants reach a height of about 18-24" when they finish, you can expect a half-ounce per plant. If you are only shooting for a half ounce with a 400W you can raise your expectations a little. If you have 6 plants, at a half ounce each, you could have a nice little 3 ounce harvest.