That works really well in Vegas, I tipped $5 on our first drink and then $1 per drink there after, the drinks are free while you are gambling. Our server made sure to get us another drink before she finally shed her shift and our glasses were never empty. This was about 10 years ago.
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.