I'd say "Class" can be used to refer to the types of economic problems or challenges facing people, regardless of salary. In this case, if the question is "what's the best way to pay for my daughter going to an Ivy" and not "how on earth can we afford this", they're not middle-class. If mortgage is the topic and "how are we going to ever pay this off" isn't the question, you're not lower-class.
I'll certainly agree that the lines are far from clear-cut, but I think it's best defined in a problem-oriented manner. The middle class gets these tax breaks because they can't otherwise afford to send their kids to good schools or own houses, while the upper-class can. They're facing different problems, and so the solutions are tailored differently for them. This causes confusion when you have income-to-SoL disparities in the country ($60k goes further in Kansas than in NYC), so people at different income levels can be facing the same problems.
Thanks, your last para clears up the need for a classification, here we'd probably say middle-income as middle-class is not necessarily related to earnings.
I guess as class and income in the UK are not coterminous this leads to confusion. For example I earn well below the poverty line in the UK but would be described as middle class due to my education and parentage.
Beckham, for example, would probably come in as working class - son of a kitchen fitter and hairdresser (according to Wikipedia) low academic achievement, low IQ, no titled lineage.
Here in America, I'd find it hard to classify Beckham as anything other than upper-class (upper-income, if you prefer). He's got enough truckloads of money (and his former-pop-star wife is also likely well-off) that he doesn't deal with middle-class or lower-class issues like affording his mortgage payments or saving for retirement or paying for college educations for his kids.
I suspect it's a cultural difference. Here in America, people don't really care much where your parents came from or how you did in school, they measure success and class by what you have. We haven't really discriminated on "old-money"/"new-money" lines in decades at least, and we've never had much in the way of titled lineages. We don't have the centuries of cultural history of class immobility that Europeans have (I don't mean that judgmentally, I'm just saying we don't have that history in the back of our minds).
I chatted to a younger friend about it and he couldn't believe I'd be so gauche as to classify people based on their background, parenting and education but felt class was and should only be about your income.
I'll certainly agree that the lines are far from clear-cut, but I think it's best defined in a problem-oriented manner. The middle class gets these tax breaks because they can't otherwise afford to send their kids to good schools or own houses, while the upper-class can. They're facing different problems, and so the solutions are tailored differently for them. This causes confusion when you have income-to-SoL disparities in the country ($60k goes further in Kansas than in NYC), so people at different income levels can be facing the same problems.