I want to point you to the following ;
http://www.webmasterworld.com/forum81/6023.htm
Here a discussion was started by a SEO consultation about a method to determine search volumes in absolute figures using an assumed value for the CTR, that is exactly what we want.
I suggest using the google traffic estimator as a yardstick for term performance, whether or not the CTR is correct is irrelevant IF
a default CTR is used constantly and IF the results presented by the traffic estimator are based on 1 value for the CTR.
If the last requirement is not fulfilled there maybe, according to the aforementioned topic, a possibility to apply google's API for extracting the CTR AND the amount of clicks for each term.
You can use
https://adwords.google.com/select/Tr...timatorSandbox
or my beta
http://idntools.net/bulktools/bulkGest.php