Since there is a ton of variation on cost of living and other factors by region, I just go by rough percentages because anything more is too complex to generalize. So the following is my view, which can be applied nationally or within a state/region as the outcome is basically the same. Keep in mind that after basic expenses are paid and someone can afford to invest their long term investments and other wealth are generally not included in their income despite their actual wealth continuing to increase allowing them access to loans and other monetary options that the middle class just doesn't have.
So how I see it by income:
Upper class as top 10%
Upper middle class would be 75%-90%
Middle class 25%-75%
Sure, people in California won't have the same monthly costs at 100k+ income to someone in the midwest, but the person in California who owns a home is building up a lot more wealth and options for loans/mortgages in higher amounts than someone in the midwest with a cheaper home. Renting is a different thing, but upper middle class and above have the option to buy and increase their wealth and puts them even further above middle class and below that aren't able to do the same or aren't able to save on top of the house payments.