Maybe it is a percentage? You know like the proportion of a book you are allowed to photocopy . After all a few words in a haiku are a different matter to a few words in LOTR.
Oh Saucepanadan.. I think you will have to class this as pro-bono... and you weren't actually instructed.... :P but it is appreciated... so interesting.. maybe I did miss my vocation after all... (but my mamma was relieved.. she felt there was quite enough lawyers in the family..
).