OK, just to take this to it's absurb extreme...

First off, assuming they round down (which remains to be seen with future miles postings), you need to tip as follows to get that elusive mile:

7.5 miles/$ is much more complicated. You have to round to multiples of $0.1333..., but in whole cents. And to complicate matters further, the cycle repeats over three times over two dollars.

Forget the theory, just always tip up to one of the following figures, where x = Even Dollars, and y = Odd Dollars:

Of course, you could always tip a little more, to the nearest even dollar.

But you'll kick yourself if you tip to an odd dollar!