it should never be calculated with tax included.
no shit, in Branson, sales tax is like 20% that would add a lot to a tip.
Also I NEVER calculate a tip based on the check. IT's silly. Sometiems we run in somewhere and take 20 minutes to drink a beer and eat a burger. $5 tip per person or whatever. Sometimes we go somewhere and linger over a meal and get lots of refills and such = bigger tip even if the ticket was the same amount. Just depends.
I dilsike the idea of percentages because it leads to waitstaff feeling entitled, you will be tipped according to your performance, not according to how much I ate or drank.
Sorry if that makes me a dick.