In restaurants, the rule is very simple: 20% for good service or 15% for mediocre service. But the fact of the matter is that tipping is expected, as it is a large part of the servers' salaries.
So, to make things really easy, just double the tax, which amounts to approximately 16%.
For example, if your check is US$ 25.00:
Food & Beverages: $25.00
NYC tax (8.375%): $2.10
Total: $27.10
Double the tax and the tip should be $4.20
Therefore
You should leave at least US$ 31.30
But roundind it up to US$ 32.00 is better
And so on.
At a bar the rule is to give $1.00 per drink. That's right, if you order a beer that costs $6.00, you will pay $7.00.
As much as you may want to leave only 10% for tip (as it is the habit in most other parts of the world), remember that "when in Rome do like the Romans", specially if you'd like to have a pleasant experience.
