Thus, the "+". The article is fair in that regard - it's just an abstract curve, with no exact formula, let alone coefficients. In practice, though, when somebody brings up Laffer, they are going to claim constants that generate a curve that peaks with a low tax rate.

In one sense, you are likely right: there is probably no formal academic theory of "supply side economics. But we have been subject to hearing this interrelated line of argument over and over since Reagan.