Forget about the ratio of follow to nofollow links
Google’s John Mueller said, “Any problem your site has would not be due to the ratio of follow to nofollow links. That’s just not a thing.” (Source)
I think no one could not say it more clearly than what John Mueller has stated above.
Should you add nofollow to banner ads?
2013 August 17
This is a very good explanation by Matt Cutts regarding banner ads on your site.