Strictly speaking, lower middle means a family of 4 with an income 45-65k, so borderline Pell and above. (65k is middle-middle, that’s why it’s the typical cap for Questbridge).
Of course people usually estimate without these references and often just base their estimate on people around them, ie., a family with two parents working full time making $15 an hour would consider themselves middle class if many people around them are hour workers paid $7.25-11 an hour but would say working class in a town where average income is above 100k…
As we all know, almost everyone in the US feels they’re middle class. 
