Plagiarism is not allowed at all. Do you, in fact, mean "a fraction of plagiarism" as detected by corresponding checking software? I think this is something different, although the latter is used to "measure" plagiarism... Of course every journal will have their own policy on how they interpret the values from such software, so only the editors will be able to answer such a question.
"You can only copy 1% of the text from one source, which is equivalent to not more than 30 consecutive words." In my case, the problem was with the captions of the figures that I had asked permission to use from other works, but I took it as plagiarism.