How can I calculate an Article's Uniqueness? -need to write a php script.

How do I calculate a spun article's uniqueness? You know like in seNuke for example, it compared average uniqueness of 1 vs 6 randomly spun articles...

Does anyone know a script/ algorithm idea behind it? I have a site where my writers write spun articles for me, and I would like to automatically 'reject' an article if its uniqueness was under 30% for example. (Or make them spin it more)...

Any ideas? Thanks

Well there's copyscape but if you're refering to a script you can have on your own site you'll need something like this:

Use dupe free pro - it's free