I am sorry, but watts per gallon does not work, it is outdated. It was meant with T12 lights. When was this book published and what is the title?
On my other forum, specifically for planted tanks, we have several people that are specialists in fields. We have someone that is referred to the lighting god, he know everything about lights, and does research. He has proven experiments done that show watts per gallon doesn't work. A 15 watt T8 over a standard 10 gallon tank would most likely be low light, how ever, a 15 watt T5HO over a standard 10 gallon will be medium high light. Also, there are many books where authors will say bettas can live in a cup, vase, or small bowls because they come from "puddles." We however know this is incorrect, you can not usually use one reference on a subject. I like to always look at multiple sites/books and see if the other source agrees or does not list it.
If you really want to see the article PM me and I will link it to you.
15 Gallon NPT
1g no tech bowl in the making