18–20% is standard at sit-down restaurants.
Tipping on the pre-tax subtotal. Most card screens use the post-tax total instead.
It depends on local sales tax. On a $50 meal in a high-tax city (NYC sales tax: 8.875%), tipping 20% post-tax sends $10.89 instead of $10.00. In states with no sales tax, post-tax and pre-tax tipping are identical. Etiquette guides have historically recommended pre-tax because the tip amount stays the same regardless of where you eat.