I finally found the JCT report -
http://online.wsj.com/public/resources/documents/buffettrule20120321.pdf
As always, the devil is in the details. In this case, there seem to be two.
1) The first detail is the "baseline". Under current law, the maximum tax rate on capital gains and qualified dividends is 15%. Hence, very wealthy individuals, who get most of their income from these sources, have average FIT rates of 18%.
IF the 2001 and 2003 tax cuts expire, the rates will go to 20% for CG and 39% for dividends. In addition, IF Obamacare survives, there will be a 3.8% Medicare tax on both.
So the Millionaires tax minimum of 30% doesn't raise (30%-15%) on cap gains, but only (30%-23.8%). Furthermore, it doesn't raise any money on dividends, in fact, the excess dividend taxes above 30% cut into the Millionaires tax on CGs, reducing the calculated impact of the Millionaires tax.
So, IF you believe the other tax increases will go into effect, then the additional impact of the Millionaires tax is $47 billion. And, IF those other increases don't make it through Congress, then the Millionaires tax is much more important.
2) The second detail is a dynamic factor. They quote a cap gains elasticity of -0.7. I believe they mean that an increase in the tax rate from 23.8% to 30.0%, which is 6.2%, results in a reduction in reported cap gains of 4.3%.
This means that the actual increase in tax collected is about 4.9% instead of 6.2%.
Of course, the question is "what happened to those capital gains?" Did they get shifted to interest paying investments and taxed there? Were they simply deferred a few years, so some of them drop out of the 10 year window but will be paid anyway? A lot of "capital gains" are just earned income manipulated into a new tax category (e.g. hedge fund manager bonuses), how do those disappear? There's always the question of how far you take the dynamic analysis.