How much less taxes does the nation collect?

you are collecting much less from the upper middle class, and more from the rich. But there are EXPONENTIALLY more upper middle class than the rich, so I believe you have LOST a lot of tax money.

Have you modelled this, or just picked 15% as a nice sounding number?

Notice also this flat tax has cause the lower middle class to pay a much greater percentage relative to the upper middle class than is paid today, since these rules disproportionately help the upper middle class.