A kernel of truth wrapped in preposterous arithmetic

The Romney-Ryan tax plan has a kernel of truth wrapped in preposterous arithmetic.  Consider a graph of the relationship between tax rates and tax revenue:

This is known as the Laffer Curve, named after Alfred Laffer, an economist in the Reagan administration.  If the tax rate were 0 percent (point A), everyone understands that tax revenue will be zero.  However, many don't ponder the fact that if the tax rate were 100 percent (point E), tax revenue would also be zero, because no one would do any work.  At tax rates higher than 0 percent and lower than 100 percent, tax revenue is above zero.  Point C represents the tax rate where tax revenue is maximized.  Point B, between A and C, represents a situation where increasing the tax rate would increase tax revenue (in calculus-speak, the derivative of the Laffer Curve is positive at point B).  Point D, between C and E, represents a situation where decreasing the tax rate would increase tax revenue (in calculus-speak, the derivative of the Laffer Curve is negative at point D).  

There is nothing at all to argue about so far.  But what is the actual tax rate represented by point C?  Clearly, tax rates higher than that point don't make sense.  When JFK took office, the maximum marginal federal income tax rate was over 90 percent.  He lowered the maximum marginal rate to 70 percent.  Although it's hard to determine cause and effect, it's plausible to assume that a marginal tax rate of 90+ percent put us on the right side of the Laffer Curve.  So JFK's change probably increased revenue.  

Reagan lowered the maximum marginal federal income tax rate to 28 percent, while eliminating many deductions and equalizing the tax rates on wages and long term capital gains.  

Did that place the country on the left side of the Laffer Curve?  A decade later, in the Clinton administration, we ran an experiment that pretty definitively proved it had.  Clinton raised the maximum rate back up to 39.6 percent; the economy boomed, and the federal government ran large surpluses.  Bush 43 lowered the maximum marginal rates to 35 percent on wages and 15 percent on long term capital gains; the economy struggled and the federal government ran large deficits. That suggests that if we raised current rates, tax revenue would rise, and if we lowered them, tax revenue would fall.  Most credible estimates place point C, the peak of the Laffer Curve, at 50 to 70 percent.

Romney and Ryan, however, live in a fantasy world where the the Clinton/Bush experiments never happened.  If JFK could reduce tax rates to 70 percent from 90+ percent and see tax revenue increase, then, well, tax cuts always increase tax revenue, regardless of the rate you are starting from.  That is preposterous.