300k is usually considered to be upper middle class, if not upper class. The threshold for being in the top 1% of earners is 420k (https://www.cnbc.com/2018/07/19/what-you-have-to-earn-to-be-...).
If being an “earner” is a major source of your income, you aren't in the upper class (haut bourgeoisie) in a capitalist society.
If being an “earner” is the major source of your income, you aren't in the middle class (petit bourgeoisie) either, just a (potentially highly paid) member of the proletariat—a wage laborer—even if you might happen to be from the proletarian intelligentsia rather than a manual laborer.