Thoughts on CTR on both PPC and SERPS

I’ve been trying to get my head around what kind of CTR I can expect to my website for both the PPC advertising that I use and from my natural position on Google Search Engine Results Page.
I would really appreciate the thoughts of anyone on this forum with a better understanding of this than I.
I found some metarial on the web that gave the following click through rate based on SERPS position. It is based upon a total of 20,000 serches for a specific keyword and I have rounded the figures up or down.
Position Clicks
1 42%
2 12%
3 […]

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