Always ignore the similarity report. The percentage will depend on the nature of your dissertation. Does it contain a lot of quotes, have you said things that are fact and can't be worded any other way?
Your uni might get a flag for similarity, but they will probably read the report and check what is highlighted. If you've correctly cited all sources and credited all quotes etc, you'll be fine.
I wouldn't worry about a similarity report unless you actually copied your work.