Rex is trying to find a better formula that applies to all tank sizes, it is by no means 100% accurate. From personal experience 70w over a 10g tank would be excessive. My tank at work has 64w over 12g and it is high light, could grow anything I threw at it and I've tried some pretty exotic plants with it. The growth rate was very fast, requiring almost weekly trimmings for the stem plants. I personally feel dropping to 50w would have been a better choice after my experience.