That's the exact book its from. I was actually going to post another thread asking how to use those percents haha!
I was thinking that the percentages were kind of vague too; which really surprised me because the rest of that book is damn detailed.
Thanks for the explanations, I've only brewed a few times but I can already feel myself becoming an addict!
Percentages are the best way to describe recipes in a universal manner. This way everyone can adjust their grain bill based on their batch size and efficiency, and their hop bill based on utilization. All you need is the target OG and IBU's to figure everything out.
The only place percentages don't work very well is describing late additions for hops. That's because you need to account for both the IBU's gained (which could be substantial if you're referring to a commercial IPA), and for the flavor/aroma coming from the late hops. One way around that is to add all your IBU's at 60 minutes and add all your late hops after your wort is cooled to 170F or so so that you get negligible IBU's from them. But that's not necessarily going to get you the same beer - it's just going to make the numbers work out.
In the end, like most things in homebrewing, you just want to get yourself in the ballpark with your first shot at a recipe. From there, keep detailed notes and adjust to taste.