This is a helpful article from last month. Why would you be interested in estimating Google organic search visits? Because of the new way that iOS (iPhones and iPads) implements Google search, which strips our referrer data. The result is that Google organic doesn’t get credit for the actual amount of traffic it yields
A typical site that generates 25% of its traffic from mobile is already seeing its recorded Google organic search visits running about 12% lower than the actual volume. If iOS 6 adoption were 90%, that figure would increase to 16%.