You're missing one of the most important -- and in my opinion, the coolest -- aspects of the system! When a missile is fired, it's trajectory is calculated and the system figures out if it's heading toward a populated area. If it's not headed toward a populated area, it just lets it land. If it's going to hit a populated area, it fires an interceptor.

The 90% success rate they claim is from testing, I believe. But they would have you believe that if they targeted 197 rockets that they took down 177ish. The rockets being fired from Gaza cost very little (a few hundred dollars a piece) vs a high tech $90,000 USD interceptor that cost hundreds of millions to develop.

In terms of country size, it stands to reason that 353 missiles, or about 65% of all missiles fired into Israel land in unpopulated areas. I don't know much about where the population in Israel is, but they apparently want to deploy 9 of these batteries and that will apparently cover the effective range of the rockets fired from Gaza.