I use to look at the tax amt and double for a standard tip. Where I'm at, the tax is now a shade under 11% now, so forced-math. Also, when in "large" groups (sometimes only 6 qualifies) they auto-add a gratuity of 18-21% (pending restaurant). Which caught a few by surprise, as we had split bills and still put it on all the checks as well.

Work's limit on tips is 18%, as it seems employees are "more generous" with someone else's money