Re: more complicated than that
That = leaving 20% of the non-tax part of your dinner bill.
There is nothing more complicated within the realm of tipping than leaving 20% on the non-tax portion of the dinner bill.
If you can understand that you leave 20% of your bill, you have understood the most complicated part of American tipping. Nothing about tipping is more compliated than leaving 20%.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.