While we used to use watts per square foot to figure lighting (and it was 50 watts min per sq ft, not 20), with the advent of LED lighting we really don't do that much anymore. It is advisable to take the manufacturer's recommendations with a grain of salt. They often over-hype what their lights are capable of. That is why it is so important to take the advise of those actually using the products. With LED lights, the stated wattage is often not the working wattage. For instance, I have a 700W light that actually only uses about 300 watts. And I can tell you that there is absolutely no way that that light would adequately cover 15 sq ft.
Your lighting is the most important component of your grow space and it directly affects the yield. Without adequate lighting, the best genetics and the best care and nutrients will be a waste.
How many plants did you start? A grow space that is 2.67 sq ft is really not large enough to grow even 1 plant. I would never ever go smaller than 4 sq ft....and even that will be problematic.
I know that I can sound harsh, but I just don't want you to spend 4 months trying to grow a plant (or plants) in an environment where it is not going to work. There are certain things you really need to pull off a successful indoor grow and good lighting and space are 2 of those things.