There isn't a set percentage that is acceptable. Please note that Unicheck may sometimes also recognise your referencing as copied and pasted, however, if the referencing is done correctly you will not be marked down for it.
We would advise that if your similarity index is high and you want to reduce it, check that: (a) Quotation marks (“...”) are used around every quote and the source is cited, (b) you are not over-using quotations, and (c) your own words are not too similar to the original text.
Some Unicheck settings create a high similarity index that you can’t fix, for example when Unicheck matches quotations, bibliographies, and strings of commonly used text (5-8 words long).
If direct quotes are surrounded by double quotation marks (“...”) and the author is acknowledged, generally, Unicheck will ignore this text. However, some Harvard referencing styles use single quotation marks (‘...’). Unicheck recognises these as text matches rather than quotations.