Is Mitt running for president again?

That is a very poorly-thought out assumption -- the basis for your "projections."

Indeed. You can tell me what the world was like 20 years ago. But unless we have been in a state of frozen since then, no, you can not tell me what your tax does in terms of FULL COSTS TODAY.

The world, and as a consequence the markets whose price discovery / equilibrium you are suggesting we tinker with with your tax, has changed in the last 10 to 20 years -- in terms of their supply / demand dynamics. The things each of us use every day differ today than they did even just 10 or 20 years ago. In endless ways, from the smart phones everyone carries (and the components that go into making them) to the use of industrial materials that have grown the emerging markets in that time.

Demographics have changed. Populations have grown. Which has changed the demand for all kinds of things.

There are materials markets that are very liquid, and are for things that are highly demanded today, that weren't in as much demand 20 years ago.

What's more, there are likely effects that your "I can project. It's simple. It doesn't hurt anyone," third-grade projection misses. As populations have grown, so has the demand (and supply) of food. If your tax drives up the price of, say, animal feed, and as a result, it makes milk more costly, you not only drive up the cost of milk, you do it for way more people than there were 20 years ago. And congrats, you just created a regressive tax. Did you work any of that into your cocktail napkin projection? You were right, by the way. It is ok to say, "I don't know," rather than selling something dangerous with simplistic reasoning.