Delivery: greater of $5 or 20%
Drinks: greater of $1/drink or 20%
Cheap Dining: greater of $1/table visit or 20%
Fine Dining: 20% before tax
Exclusions: table visits due to server error or goods not received
Outstanding: additional tip, preferrably as cash, above and beyond the standard.
Perfect: tip rounded up for easy math.
Standard: 20% of the undiscounted price of the goods received.
Bad: Tip-O-Meter decreased to no less than 10%.
Horrible: Tip-O-Meter hits zero, and a placeholder is left such as a penny or a banana.
Outstanding means the server is socially engaging, charismatic, polite, and performs functions accurately. This ties "Bad" for the third most common rating -- maybe yearly, though I might visit this place several times afterwards, forsaking other places which some might consider that as making this a tie for Standard (visits rather than places visited).
Perfect means all functions were performed as requested and expected, but nothing extra. This is the second most common rating.
Standard means everything was generally OK. Overall feeling is neutral. Minor issues were resolved. This is the most common rating.
Bad means more than one thing was not received as ordered, or errors were not corrected in a timely manner. I'll probably visit again, so I don't want them to spit in my food. This ties "Outstanding" for the third most common rating -- maybe yearly.
Horrible means multiple problems, including server ignoring door and/or tables to socialize with friends, a single food item is incorrect even after request for correction, offensive attitude, manager is unresponsive to issues, etc. I will probably not visit again. This is the least common rating -- maybe every couple of years.