0% if you're poor and mean, or just mean.
15% as a bare minimum if the service was below-average. Some people just do this all the time though.
20% if it was good.
Up to 30% (and beyond) if it was amazing.
Tax is never included when listed in the advertisement, label or menu. It is layered on at multiple levels, and sometimes invoices show this. Here the taxes are:
Washington State: 6.5%
King County: 3.5%
Seattle city: 0.1%
(Supermarket raw ingredients are not taxed, which is normal in the other places I have lived.)
This makes calculating tips in Seattle easy - just double the tax on your bill. However travel outside the Seattle city zip code (e.g. go to a satellite city), county or state, and your tax, total at the till, and the tip you must calculate change.
It's all become a bit much for me and I've resigned myself to just paying whatever the new total is.