A tip is a numeric sum based on the standard of services rendered by a server and staff during a meal. When dining at an upstanding eatery, a meal served in a timely, appetizing, and helpful fashion deserves a tip of no less than twenty percent.
Paying for a meal includes tip. There is to be no splitting, credit for cash, cash for credit, or other shady dealery to be involved in with your gastronomic lady friend. Save the haggling for the Brooklyn Flea.
When purchasing a bottle of wine that is significantly more expensive than the average cost of the entrees, reduce the cost of the bottle of wine to the average cost of the entrees.
And of course, if your server is entirely subpar, tip them accordingly and feel free to speak to the manager.