SERP Positioning & Percentages

From time to time certain forum posts get my attention, if not for their unique perspective, but their inability to be properly answered. The most recent post from DigitalPoint covers the probability of result pages and their click throughs.

I want to know out what percentage of clicks the 1st position gets vs the 2nd position vs the 3rd position, ect. for the top 20 SERPs

example:

1st position 68%
2nd position 52%
3rd position 55%
4th position 31%

Does anyone really know?

Unfortunately this post was made at the beginning of September without any responses and has recently been bumped. Still no response...does anyone have any better evidence of click throughs?