@roger,
Here's an example of the extremes that "can" happen in high end restaurants.
Say the bill for four comes to $1,000; the tip at 20% is $200 for one table. Serve two or three tables, and you're really making some big bucks.
On the other hand, work at a mom and pop family restaurant where the typical bill for four comes to $60. A typical tip will be around $9 at 15%. Whether it's part time or full time, it depends on the success of the restaurant. At the lower end restaurants that are open 24/7, they probably use a lot of part-time help during lunch and dinner hours, and not much beyond that - unless they do a good breakfast crowd. With the average bill in the $15 to $20 range for dinner, that's a $2 to $3 tip. My wife, son and I went to a Chinese restaurant for lunch today, and the bill was $24. I left a $4 tip.