Cap spending at 5%.
The whole BBA exercise is a farce, and tax revenues historically don’t exceed 18%, which is far too much to give the swine in D.C.
It depends on what taxes you include.
This is all taxes: individual and corporate income, social insurance, and excise. Note that the tax rate has hovered just above 20%.
You can find graphs of average tax rate, by the type of tax, at: US Federal Taxes by Income Level. They were derived from data published by the Congressional Budget OFfice -- which is also archived on that page.