Why would that be frustrating? The accountant tips 15% for average service, you tip 15% for average service. It's based on the service you receive, not the financial standing of the person who is paying. It should make no difference to the server whether he helps those on expense accounts or a non-stop series of poor students. As long as he provides good service, he should end up with the same check at the end of the day.

If it's really that confusing, just stick with 15%. Keep that one number in your head. A phenomenal server might question herself why you didn't perceive her to be going above and beyond most other servers, but she won't be angry that you didn't tip her more. 15% is not a hard number to keep in your head and is indeed less confusing than it is for Americans told to "just round up" in other countries--round up to what? 9 gets rounded to 10? 18 gets rounded to 20? 15 gets rounded to 20, too, or only 16? 47 goes to 48 or 50? 61 to 62 or 70?