Re tipping rates: the sales tax rate in NYC (and many other large cities) is in the vicinity of 8.5%, so doubling the tax = 17%, was an easy way to compute the tip (and possibly over-tip when the rate was 15%).
A recent experience: The waiter, in error, placed my salad order late, and so the main course and the salad arrived at the same time.. He neglected to provide a steak knife (I had to request one from the server). I gave him a 15% tip (which was more than he deserved--a waiter is supposed to provide competent service). Question: I was thinking of attaching a note to the bill (and tip), noting why I had "under tipped" 5%--should I have done this?