I wasn't meaning to imply that wealth should be taxed, however it's a fair point that I'm rather badly conflating income and wealth in the OP - I'm basically using wealth as a proxy for income. I suspect that they are very strongly correlated though (a quick google shows [1]). Unfortunately I can't seem to track down a hard figure on the 20th percentile of income in the US.

[1] http://www.visualcapitalist.com/relationship-income-and-weal...