If tax revenue is a continuous function of tax rate, then according to the mean value theorem there is a certain percentage between 0% and 100% at which tax revenue is maximized. Call it m%.
I hear this argument a lot. However, you (and Laffer) assume a specific shape for the graph implicitly when you make the argument, which isn't supported anywhere except right close to the 0% and 100% mark. It also assumes extremely simplistic single-good and single labor supply economy, which isn't real. The relationship of revenue to tax rate isn't linear. None of this is nearly neat enough to fit any U-shaped graph. And since it isn't a simple equation, you can't use the "m% theory" to argue whether we're ahead of or below the optimum tax rate in any practical sense.