Set the single tax rate on all remaining income so that it exactly replaces total current IRS & Social Security revenue. Consolidate two separate bureaucacies into one for greater efficiency, but maintain separate Soc. Sec. trust fund.
This system eliminates the marriage penalty & allows families with adult children living at home to file a single tax return covering all household members, thereby eliminating redundant, wasteful paperwork for both citizens & government. It also makes the rich pay at the same rate as everybody else, instead of the lower rates they enjoy today due to creative classification of their income.