I looked it up. The middle class is typically defined in the range of 20th-60th percentiles, and constitutes about 50% of the US population (which is lower than it has been in decades). It's a bit squishy (e.g. why not 25th-65th?), but that's the consensus.

The more useful numbers are "who owns the wealth". The top 10% owns 70% of the wealth, and the top 1% own half of that (35%). The top .1% owns half of that (~16%).

So it is clear where we need to focus our tax efforts/enforcement. A minor quibble, though. You said "at that income level". These are folks who own the wealth, not necessarily who have the highest incomes. Many of them don't work at all, they just rake in dividends and cap gains. As a TMF poster used to say, the worst way to get money is to work for a paycheck. The best way is to let the money do the work, and take advantage of the privileged status that money holds in terms of tax law.