How much new content do I have to add to the wikipedia source to avoid
being marked as duplicate content and penalised in my SEO?
That question may raise a few eyebrows, as it appears as though you are asking what bare minimum percentage of change is necessary to avoid spam detection (which raises the question of whether or not you are attempting to produce undetectable spam).
You may want to consider Google's own Duplicate Content guide and Jill Whalen's Duplicate Content Penalty Myth article (which echoes Google's recommendations) - fair use of content from other sources will not trigger a penalty of any kind, and, though it would be inadvisable to copy things wholesale if you want to rank first in results, there is proof* that Google does not penalize such activities.
(* See answers on that question for active, attribution-compliant clone sites or try an exact phrase search for any StackOverflow question to see how Google prioritizes duplicated content)