If your butt is still in your seat when the restaurant closes, the amount you are ethically obligated to tip automatically goes up to 20% of the post-tax total. For every two minutes you remain after closing, your tip obligation increases by 1%. So, for example, if you stay thirty minutes after close, you should tip a minimum of 35%. Don't want to tip 35%? Don't stay thirty minutes after close.
Your friends in the service industry.