"The second element would prohibit Congress from spending during any fiscal year more than a fixed percentage of the GDP of the previous calendar year. Although people may disagree on the proper percentage, I would put the figure at 18 percent. Over the last forty years, federal revenue has averaged roughly 18.5 percent of annual GDP, so a constitutional amendment imposing an “18 percent of GDP” limitation would require Congress to live within its long-term means, thereby preventing the inexorable expansion of federal spending relative to the size of the economy. (As of 2011, federal spending amounts to a staggering 25 percent of GDP; to put that in perspective, a century ago, both federal spending and federal revenue stood at just 2 percent.)
"This limitation would stop Congress from manipulating revenue estimates. In fact, such estimates wouldn’t be used at all, since the previous calendar year’s GDP would be calculated before Congress adopted each year’s budget—and that’s a fixed number that could not be gamed after the fact.
"That number could, however, be manipulated beforehand, by changing the data points that have historically been used to calculate GDP. But it would be relatively easy to detect that kind of machination and to identify those responsible for it. Once the data points have been ascertained for a particular year, GDP can accurately and reliably be determined. And given that the GDP figure in question relates to matters of historical fact, it would be difficult to manipulate. ..."