Is $21 Trillion Really Sheltered "Offshore"?

In a white paper that is getting some traction on the web, primarily due to write-ups in the Guardian, a group known as the Tax Justice Network estimates that $21 trillion dollars is being sheltered "offshore" by wealthy individuals and companies to avoid taxation.  There is no doubt that substantial amounts are hidden in tax havens to avoid taxation. Yet the white paper, which I've read, is not highly credible in my opinion.  The Tax Justice Network, having gone through their site, looks like a somewhat fly-by-night operation to me; some of its supporting materials are as much polemics as anything. It's blog appears primarily devoted to advocacy. The issue of unfair taxation and substantial avoidance of taxes, based on the laws on the books, is an important one, and this calls for a detailed analysis by a group such as RAND, the Brookings Institution or Pew Charitable Trust. Heck for all I know they may have already done one.