It's important to think of the green line as the
SUM of wages paid to workers earning from LOWEST to HIGHEST wages. It's called a Lorentz curve. (See
http://economia.is-there.net/the-lorentz-curve/ ). Thus, straightline arithmetic on wage-levels yields non-intuitive (and probably meaningless) results.
All workers who paid OASDI taxes on their ENTIRE wages (i.e. they earned $90K or less) received just shy of 70% of all wages paid - and they were about 94.1% of ALL workers. In other words, only about 5.9% of workers earned over $90K - wages not subject to OASDI taxes because they were over the 'cap.' (But they together received over 30% of all wages paid.)
Q: "
What is the dollar value where the green line intersects with 50% of total wages?"
A: That's about $57,500. Again, workers earning up to about $57,500 together received half of all wages paid. But that's over 85% of all workers. Less than 15% of all workers together received the other half of ALL wages paid.
So, if "middle class" means "bottom half" of all wages paid, then I suppose one could say it's up to about $57,500. I wouldn't, but one could.
To give an example of how straightline arithmetic doesn't work for a curvelinear series, consider that (in that chart) 97.87% of all workers received wages under $140,000. Together, they received 81.3% of all wages paid. On the flipside, 2.13% of all workers received 18.7% of all wages paid.
Further, 99.53% of all workers received wages under $300,000. Together, they received 90.1% of all wages paid. The remaining 0.47% of all workers together received 9.9% of all wages paid.
Those at the top get lion-sized shares of the wage pie. (That's what a Gini ratio describes. That's what the nearly vertical slope at the right side means - and the flat slope at the left.)
Remember, if EVERYONE GOT THE EXACT SAME WAGES then the green line would be straight, at 45 degrees from the lower left to the right top. The 'sag' in that line portrays anormous disparity in wages paid to workers. The whole question becomes 'how much sag is too much?' The 'sag' is only half as much for European countries and Japan.
It takes a bit of straighforward interpolation from the data provided by the Social Security Administration (which they obtained from the IRS) to determine these boundaries. When I've tried to examine the IRS data, it's been obfuscated (using IRS smoke and mirrors) to the degree that I can't easily extract better than what they provide the SSA for calculating the AWS (used to make year-to-year adjustments in benefits, wage cap, etc.)
Sorry about the edits ... eyeball parallax. (I misread some figures in my spreadsheet and had to fix them.)
PLEASE NOTE: The reason I figure 84.8% of all wages are subject to OASDI tax (instead of, say, 70%) is because those workers earning over 90K still pay OASDI on their first $90K. Thus, that amount must be added to the 70% figure (which relates to the TOTAL wages paid to workers whose ENTIRE wages are taxed).