What is an acceptable percentage of matching text in Turntitin?
The overall percentage of matching text in Turnitin is an accumulation of all the individual percentage matches, regardless of whether they are small or substantial matches So there is not really an acceptable percentage because this value can be arrived at in different ways.
For instance, numerous small matches can result in a large overall percentage which is relatively unimportant, but a small overall percentage may be the result of one or two copied paragraphs, and this is more significant. Therefore, you should examine all matches and understand why Turnitin has highlighted them. It is possible to reduce the overall percentage by excluding quoted text, small matches, and the reference list.
For more information you may find the TurnitinUK page useful.