Lets look at the nurabers using Joe's perfect world scenario.
A person making $20,000 will have to pay $3600. That leaves them with $16,400 a year take home.
A person making $2,000,000 will have to pay $360,000 leaving them with $1,640,000.
Who is worse off?
Who can still save at least 35% of their income and still live comfortably, and who can't? Thus resulting in less money getting put back into the economy.