As I have seen some papers got 10% plagiarism where people start calling them scammer, I think Less than 15% plagiarism should be allowed [which is allowed worldwide] as there are many cases where one can not change some terms, as if they do they'll completely change the meaning.
Problem with an average whitepaper we see here is not that few terms here and there are copied, but whole sentences, paragraphs and sometimes even pages are copied word for word from someone else, and for that there is no excuse. But even if they just copied few sentences, how hard is to include reference? For example, it can be done in an unobtrusive way, like in Bitcoin whitepaper.
I simply do not trust those that are not capable of writing their own whitepaper without proper referencing. In best case scenario they are just ignorant and negligent, while in reality they are deceitful and dishonest. Neither is good and will most likely cause people loose money.
By the way, do you have some specific case in mind when you are talking about percentages, as I know that you are creating ANN threads for various projects?
For your work to be as clean as possible, your plagiarism ratio should not be above 10% - 12%
Well, for your work to be as clean as possible, plagiarism rate should be at 0%, and that means that you cited every source used, and you made clear distinction between your own words and what you took from the others. So just verbatim copying something and sharing links at the end isn't proper way either.
Similarity index is another story, and there we can talk about percentages and what is good and what is not.