By nature, I am not a big tipper. I prefer the "service charge", which still allows one to add more if the service is outstanding.
When I lived in Britain, I generally conformed to the practice of tipping 10% of the bill, somewhat rounded up. Nowadays, I conform to the usual American practice, in which 15% is the standard (or at least it used to be: there is some "creeping inflation" pushing the level up to 20%, which I tend to resist.)
On a recent evening, I came to pay the bill for a party of twelve at a family dinner. The menu clearly stated that a gratuity of 18% would be added to all bills for parties of 6 or more. (I understand why restaurants do this, but that doesn't mean that I like the practice) To my surprise, when the bill arrived, no such charge had been made. I didn't have a portable calculator in my pocket, and so I multiplied the total shown on the bill by 18%, and wrote in that amount as the gratuity. I smiled at myself: Why did I do that? I could have simply asked the waiter to add the 18%. Again, with the amount of the tip left blank, why didn't I just add a tip of 15% or thereabouts? Truthfully, the service had been slow and spotty, although quite "professional".
In earlier years, I used to become quite irked when expected to include the tax before computing the tip. What did the governor's impost have to do with the service we had received? Going back several generations, I am half English and half Scottish, and the stereotype of Caledonian frugality has occasionally been justly applied to me, although I have mellowed on this with the passage of time.
On one occasion, about thirty years ago, I gave a substantial tip in advance of the meal, with the promise of more to come at the end. Barbara and I were entertaining a friend who had an exaggerated idea of his own fame as a painter. I coached the waiter to pretend to recognize the man, and to flatter him almost to the point of the artist's realization that he was being "set up". Perhaps the waiter was an actor "resting" between engagements, because he engaged in this jest magnificently. Barbara and I sat giggling to ourselves, and the man never caught on. I don't consider myself a vengeful person, but this was my payback time, since our friend had told Barbara that I was "irascible". That adjective has been featured in Barbara's comments whenever I speak wrathfully--and she certainly has a point!