By analyzing more mail/holdout tests than I care to mention, I have a really good idea how catalog marketing drives demand to (or cannibalizes) other channels.
You might observe, for instance, that 40% of all search purchases were caused by catalog marketing (not matched back, mind you, but caused as measured in a mail/holdout test).
And you might observe that 90% of telephone orders were caused by catalog marketing.
Well, then you have something, don't you?
You can take a segment of customers at the start of 2010, and you can measure the percentage of demand spent during 2010 by channel. Take 40% of search, 90% of telephone orders, get the picture? That's your organic demand. As a percentage of total, you have your organic percentage!
Now, I use more sophisticated methods than that ... I do this stuff at a customer level, with models that combine prior channel preference with Digital Profiles and the like. I have time-honored and tested tricks that cause the outcome to be more robust.
But in general, it's that simple!