I find etiquette fascinating, so I've read some books on it. Particularly, in tipping etiquette, prescriptive books seem to agree about one thing:
In the US, if you're going to tip less than 15%, you should not tip at all. You should leave nothing and ask to speak with a manager to explain why you won't. If the failure in service doesn't rise to that level, then 15% is the minimum polite tip.
Of course, it's your wallet so do what you think is best. But this seems like a reasonable practice, so I follow it. I've actually never had to speak to a manager about service at a restaurant, though.
The books are not correct as far as practice goes. 10% was standard for years and 15% for great service. Waiters of course have been trying to promote 20%, 22.5% and more as normal.
10% is not great, but is what senior citizens and rich politicians will leave if they tip at all.
10% is considered an insult in California, and will be treated as such.
I accidentally left a 10% tip at Fisherman's Wharf (Early on in my california experience), and was approached by the Maitre D as I was departing, asking what was wrong.
If you can't afford to tip, best just not to eat there in the first place.
I've also noticed that many lower-end restaurants will print pre-computed tip values at the bottom of the sales receipt for 15%, 18%, and 20% (both as a prompt and as a "nice" thing to do so you don't have to do the math yourself). But these values seem to always be calculated off the post-tax cost of the meal, whereas convention seems to be that you tip based on the pre-tax subtotal.
I like to keep it simple. 20% or 25% for fair to good or really great, 15% for poor service. I too don't like how people are trying to push this value up. But at the same time I have enough and I don't mind sharing some with a person who has just served me dinner.
You can do it pretty easily by successive halving.
10%, just move the decimal over
add 5% which is just half of 10%
add 2.5%, which is half of 5%
and add 1%, moving the decimal over twice
...of course you either naturally think that way or it seems like an unreasonable amount of work. YMMV.
Personally, I usually take 15% and add a bit depending on how good the service was.
In the US, if you're going to tip less than 15%, you should not tip at all. You should leave nothing and ask to speak with a manager to explain why you won't. If the failure in service doesn't rise to that level, then 15% is the minimum polite tip.
Of course, it's your wallet so do what you think is best. But this seems like a reasonable practice, so I follow it. I've actually never had to speak to a manager about service at a restaurant, though.