Wait staff in the US are definitely underpaid... and they're also taxed on what their tips *should* be. IOW, if they did $100 in business, they should have received $15 in tips (or whatever the magic multiplier is) and are taxed on that.
I generally tip from average to well (average being around 15%). Sometimes, it doesn't make sense to figure it out mathematically (15% of 4.35? Heck, it's a buck or a round-up).
One of my important rules: if I'm going to come back, a good tip is a wise investment in my future