What's your purpose in doing this?
You want duplicate articles that are almost the same? Why?
I think if your purpose is to out-wit search engines, you can never have too many words different, and you can't ultimately succeed. They will just keep getting smarter.
The problem with any kind of deception on the web is that it's "on your permanent record". Once discovered, it can turn against you quickly and permanently. There is no way to extricate yourself.
Write for your readers, and you don't have this problem. Do your readers *want* duplicate articles? Probably not.
Of course, there can be many reasons why you want to rewrite articles. Perhaps you've gotten them from somewhere else, you're putting one copy on your site, and don't want to be penalized because it's a duplicate of an article found elsewhere.
Fine. Rewrite them for *your* readers. Don't worry about word counts and such. Take those fine articles (cough, cough) and actually tailor them for your audience, and the problem will take care of itself.