My rule of thumb is that at a bar, if a beer costs less than 6 or 7 dollars, it's a $1 tip, more than that $2. Cocktails get a bigger tip, depending on the complexity of the drink, but typically land somewhere around the 25% rounded up to the dollar mark.
If you're on a tab, you tip at the end, but a slightly larger cash tip on your first drink will get you much faster service the rest of the evening.
They were tips on free whiskey/vodka + mixer as we were gambling. I googled at the time and those numbers were recommended for that service. For food drinks etc would normally tip 15-20% depending on service, would also go lower if it was terrible.
Having people rely on some unwritten custom where the rich are expected to give a handout as part of their meal feels much more like treating people as slaves than simply paying people a fair wage and charging what that costs to make workable.
I know it's just a different culture and I try to give a decent tip when I'm in the states as my lack of understanding will not improve the situation but it's still fubar.