Isn't it three plants?
I've explained how I reached the estimate I've given. Most new growers yield 0.4 to 0.5 grams per watt with reasonable exception for high yield strains like Chronic or OG #18. There are a lot of factors involved with yield predictions which makes it really just a guess in the dark. I pulled about 240 grams from my first 400w harvest in Fox Farms soil using their nutrient system and was unimpressed. A follow up grow in coco yielded about 340 grams despite some setbacks.
Considering experience, genetics, system, environment, and lighting... hitting 0.5 grams per watt means you didn't blow it. You didn't quite nail it though. Short of CO2 supplementation and environmental controllers ($1400 minimum investment) breaking 1.1 grams per watt means the grower has quite a bit of experience with a high yield strain or just got damn lucky. Many growers do break 1 gram per watt without CO2, but this usually involves a fairly savvy vertical system to increase canopy size and minimize energy loss. The largest 400w grow I've ever seen was Serious Seeds Chronic grown in a Hempy style drip system that incorporated hydroton and rockwool mini cubes in place of perlite and vermiculite. The grower used Canna Substra drain to waste and did a single plant in scrog yielding 450 grams. The grower had been working the same strain in the same space for years, and this was his best harvest as he'd usually pull closer to 10-12 ounces.
So, for new growers I say to aim for 0.5 grams per watt. If you happen to get more, and you easily can, then woo hoo! Expecting to hit 400g on a 400w bulb when you've never done it before (you have to ask what you're yield will be) is just a set up to be disappointed. Any grower worth their salt with some experience with their signature strain should hit 10 ounces without much trouble.