I’m under the impression that “middle class” vs “working class” and “upper class” was conventionally more about labor relations.
If you derive your income from ownership, you are upper class. If you derive it from the value of your work work, you are working class. In between we have a class of people with high-value professions and/or assets such that they are basically able to make their own employment decisions without any real discomfort. Merchants I think were the go-to historical example, I’d put highly paid contractors in that class nowadays. This class is literally the class in between the of the other two, aka the middle class.
If you own a company that employs people, you are upper class.
If the boss (maybe the person contracting you) asks you to do something unethical or dangerous and you tell them to get bent with 0 worry as to your next paycheck, you are middle class.
If the former scenario happens and you consider doing the bad thing because you’d rather not be homeless, you are working class and should probably think about forming a union or something.
Depending on your expenses, you could be middle class at 90k and working class at 600k, but if I was working class at 600k… I’d look into getting my expenses under control…
Often in the US we say “middle class” when we actually mean “median income.” It isn’t great.