Q: Where can I find information on how your SERP converts to a traffic percentage?

Dear Kalena...

I was hoping you may have some advice, the SEO forums have had no answer to my problem.

I have been asked by a potential new client to come up with this information for his board of directors if at all possible. So I am asking for help! Do you know of any papers / articles / threads / info that discuss how your SERP (and even more to the point your exact position in the top 20 resuts, position 1 thru 20) converts to # of visitors in a percentage basis?

For example. I have a search term that occurs 10,000 times per month as reported by Overture. If I am the 20th result for this, what % of those 10,000 searches would I get? If I am the 10th results for this, what %? and so on to If I am the 1st result for this, what %?

I realize this isn't an exact science but if you know of research results in this area it would be a HUGE HELP to me.

Thanks!
Glenn