Basic calculation for all you Math legends - please help

OK,

I've got a question which is mega complicated for me, however thought if anyone could help it would be you guys.

I am going to be submitting an article to say 2000 article directories, now to get maximum exposure from the search engines, the article needs to be as unique as possible. Now lets say that the % of uniqueness I am going for, is 40% unique.

Lets say I have 20 different sentences and for each sentence there are 5 different options, so 20 x 5. The variables of the article get choosen completely at random, so taking this into account what are the odds/% that I will get a 40% unique article each time?

Having a formula would be great, because then I would be able to work out whether I need to increase sentences and options for sentences.

Thanks in advance.

Adam