In a high percentage of restaurants I've been to recently, one person takes your order, a different person brings you your food, yet another person checks on you periodically (or you have to flag someone else down if you need something) and you end up paying your bill using a device at your table or at a lobby register. Servers do way less now with virtually no customer service emphasis but expected gratuity has skyrocketed.
My current process is... I'll tip my waitress/waiter ~20% if they take my order, bring me my food, check on me periodically, offer refills & deserts and handle my payment - you know, full-service waiting. That's my baseline. I might add a percentage if the server offers some kind benefit to my party (such as free ice cream to my daughter because it's her birthday). I knock percentages off the tip for anything short of full-service waiting from the one person who is supposed to be serving me. If I'm eating at Red Robin, my tips are always low because the server who is supposed to be serving me does next to nothing for me. I walked out of the Oshkosh IHOP earlier this year providing a $0.00 tip because they absolutely earned it, but normally if they at least perform their duties adequately the lowest I go is between 5-10% - unless there's automatic gratuity, then I factor that into my percentage.