Just tell me what's expected. If you want me to leave 18-20% in cash, I will. if you want me to add 15-20% to the bill and do the math, I will. If the restaurant wants to add 18-20%, that's fine I'll sign the bill. At least in the US, service has little to do with the tip, it's an additional charge that's expected. I'm not going to try and re-invent the system because I think it should be different.