I'd say 20% became the "new 15%" in metropolitan areas more than a decade ago; I'm not being rude, I'm just dating the change.
I live in a major metropolitan area. I don't have a set percentage. I look at a couple of things. 20-25% is my norm, leaning to 25%. If the service is worse - service, not the food - I leave less. If I get particularly good service, I'll leave 30% or more - but only if I've asked whether tips are pooled. Being a server is not an easy job.
The second thing is I have a base. I rarely tip less than $3 or $4 is the service is good, even if it's on an $8 sandwich tab.
Mrs. Scoutmaster probably tips half of what I tip (on a good day). She's a perfect example of what the server's stereotype regarding female diners (I can't say if the stereotype is true, and I'll keep silent about my experience when the tab is picked up by female business colleagues). She'll try to calculate 15% and then she'll round it down to the nearest dollar.
As an ex-waiter in a number of great places in Miami, I agree 20-25% is a good tip which will be appreciated by your server. Especially in this day and age. Round it up a little if you intend to come back to that restaurant. You will be surprised how good the service will be next time you show up! Yes, Servers have good memories, and they tell each other who the good tippers are. Changing the subject slightly, does anyone have an opinion on whether to tip tow truck drivers when they are towing you under contract with AAA etc. I have wondered about that.