I&P:
The problem is access to the source material. For example, if we know Quotes quotes, say, 500 words from the book Your Kingdom Come, we would need to know the total word count of the book in order to get the percentage quoted. If you have access to these old materials, I can give you a list if you want.
Actually, though, now that I think about it, any quotations from books are going to be very small percentage-wise. Assuming a book has at least 200 words per page, then a 200-page book has 40,000 words, and since almost all the quotes on the site are less than 500 words, we are talking about perhaps 1%. The magazines are the only real concern, and the question is, as Leolaia points out, what constitutes a "work"? Is it an article, or an issue of the magazine?
I think that, for now, we can stop worrying about this issue. I just thought it would be good to have some figures in our back pocket if we needed them, and I think we know now what we need to know.
Quotes:
Glad to help! You can have a copy of the database if you want. I put the content in a SQL Server 2000 database. It was kind of quick-and-dirty, but it might give you some place to start if you want to transition to a db-backend at some point.
SNG