The real formula is 0.5g/ each 30 days/ per watt of light in the flower room.

So if you flower under a 400w HPS for 8 weeks (standard) you are going to try to max out at 200g/month.

This is absolutely great.

But you have to optimize your light. If it is only 1 plant, you are going to want to leave it in veg for a while and train it to be wide and really catch ALL that light.

If it is 20 plants, you can cram them together when they are still small under the light.

You might reasonably expect the same yield from either setup!

For your first grow, 0.1g/30d/watt is sort of to be expected. And you will learn to improve it over time.