Dear Helena,

When, if ever, should you tip more than 20 percent in a restaurant? I’m lazy and always just tip 20 because it’s easy to calculate. —Bad with Numbers

Dear Bad with Numbers,

Most of the time, you need not tip more than 20 percent, but here are some occasions when you should be more generous:

1. Exceptional service where you wouldn’t expect it. If the waiter arranges a special gluten-free pasta dish for you, replenishes the napkin supply as your toddler shreds it into confetti, and then sings happy birthday to your wife, you should reward him. But only if the service goes way beyond what you’d expect for the type of place. If you’re dining at Per Se, you expect VIP treatment, and in any case 20 percent of the check is usually a pretty good chunk of change.

2. Freebies. If the server brings out cheesecake all around or complimentary snifters of homemade limoncello, you should tip on the estimated cost of the comped items. These might include waived corkage. Matt Fitch, who has worked in the restaurant business for 11 years and is the sommelier at Coi, says that if he allows a party to crack open a few bottles of their own wine at no extra charge, it usually earns a lavish tip.

3. Getting treated like a regular. I tip big at a breakfast place where the waitress always remembers my order and on top of that smothers me with so much love I call her my coffee-shop surrogate mother. When someone calls you “my darling,” it’s impossible to leave her $1.75 in change—it’s a karma thing. Steve Dublanica, author of Waiter Rant, says when he worked in New York City–area restaurants, some regulars tipped him 30 percent. As a result, “if their friends or colleagues needed a reservation on Saturday night at 7 and they called at 6:30, I would make it happen.”

4. A low check. Sometimes the check is so low that 20 percent seems stingy (and may involve the annoyance of fiddling with coins). Then just leave a couple of dollars or so. For instance, Zach Brooks, creator of the blog Midtown Lunch, says when breakfast is seven bucks, he’ll tip three so he can leave a tenner.

5. A “camping” tip. If you linger (known as “camping” in restaurant lingo), you should definitely compensate the server for the lost business. Brooks says, “If I’m loitering over coffee, I will tip a little more than 20 percent because I know they could have turned the table over.” Even if the place is empty, you should augment your tip a bit, because the waiter may have offered you more service while you hung out—checking your water glass or asking if you want anything else. And of course throw in a little more if you camp so long that the servers are upending chairs on tables and sweeping the floor.

6. A pity tip. If the server looks like a malnourished student living on purloined ketchup packets, you’re not obliged to tip more, but it’s nice if you do. Dublanica says he’ll tip big if the staff is “under stress and running around like chickens with their heads cut off; then I know that’s the management’s fault.” I always tip big at an upscale tea lounge near my house because the place is often empty and I’m convinced it won’t survive the recession.

Interestingly, the biggest tip Dublanica ever got—$500—wasn’t for any of the reasons above. It was what you might call a “guilt tip”: “The guy was one of these jerk ‘master of the universe’ types with a black American Express card. He came in with a paid escort, and was quite drunk. He left a 25 percent tip, then asked, ‘How was the tip?’ I said, ‘It wasn’t enough.’” Shamed, the guy doubled the amount. The lesson: However much you leave, you’re asking for trouble if you say, “How was the tip?”

CHOW’s Table Manners column appears every Wednesday. Have a Table Manners question? Email Helena.

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