Optimizing the restaurant sales receipt for maximum tips

For example, some people tip 20% regularly, but 25% tips are nearly unheard of -- AFAIK. It seems to me that your options will tend to drive customers toward 15%. (EDIT. See the comment by bostonpete for an example.) But if the three were 15%, 20%, and 25%, then I imagine you'd get a lot of people picking the middle option. Maybe even with 15%, 21%, and 27%.

Also, as a legal matter, don't credit cards require the final amount to be explicitly stated? It would probably work if you included both the tip and the final total in the option to circle. You'd also need to include a blank to write the total in, for people choosing their own tip amount.