[jyl]

Presumed tip rate for tax purposes is around 8% for federal, though it depends on the employer's tax reporting policy. Employer has the choice of having waiters report their actual tips received, rather than using an imputed rate on sales. Waiters are probably in 15% marginal tax bracket ($8K to $33K income). 15% of 8% is 1.2%. So, if you stiff a waiter for his tip on your $100 bill in a sit-down place, you "might" be costing him $1.20. Add a dime or two for state tax.

I find that I tip based on whether it is my regular place.

At my regular morning coffee place, where I chat with the same person every day, I tip $1 on a $1.80 coffee. At my favorite restaurants, I tip 20%. My favorite Chinese place gets 15% on the takeout bill. Other sit down places where I am not a regular, 15%, and other takeout places, zero.