There are far too many factors to consider if you want to tip a "fair" amount. As Jerry mentioned above, the cost of living in San Francisco is much higher. Of course, that is assuming that the waiter lives in the city rather than commuting in from elsewhere.

Then you have the fact that restaurants (as with most things) in San Francisco tend to be more expensive than elsewhere, so even if you keep the percentage the same, the tip will be higher.

Finally, remember that the city provides subsidized health insurance for minimum wage workers.

There is really no way of determining a fair tip based on all these factors. I wouldn't try to adjust tips accordingly, because you really have no way of knowing the specifics.