I commute from the back of the valley in Linda Mar into the City every day for work, and also surf as often as I can, so I get a very good look at where the "fog line" is day in and day out. First, you should probably consider that for about half the year, fog is not a big issue anywhere. From about mid-October through mid-April, it's either sunny everywhere around San Francisco, or it's cloudy and/or rainy. But the fog does play a big role in our weather for the other half of the year, and since that's otherwise the sunny half, it's even more important to many people. It's also worth nothing that the fog waxes and wanes both day to day and within the course of a day, in response to changes in the atmosphere.
That said, there is definitely a "fog zone" in the Bay Area where from April to October, on most days, for most of the day, there is a layer of fog and low clouds present. If that's an issue for you, you should definitely take it into account. The basic issue for understanding the fog zone is that the fog is sucked into the hotter inland areas through gaps in the coastal mountains along the coast. Because of that, even as the clouds burn off through the day, more clouds are sucked in behind them, so if you are in the gap, even miles from the coast, you don't see the sun, even as other places right on the beach a few miles away are basking in it.
The two big features determining the movement of fog and low clouds on the north Peninsula generally are the San Bruno Gap and Montara Mountain. The fog, wind and low clouds flow over the southwestern corner of San Francisco, Daly City, and the northern end of Pacifica on their way east to Colma, South San Francisco, and San Bruno. A little further south Montara Mountain rises 2000 feet from the sea, and redirects the fog, wind, and low clouds away from East Sharp Park and Valley Mar to an extent, and then more so from Linda Mar, to the greatest extent the further south and east you go. Over the hill, you'll notice that Hillsborough and Burlingame also enjoy the protection from Montara Mountain, though the difference between them and communities north and south is obviously not as dramatic.
So on a typically foggy day, on my commute home, I'll shiver as I walk from through the Daly City BART station to my car, and it will be foggy everywhere until I reach the corner of Fassler and Terra Nova, and then it will be sunny all across the Linda Mar Valley from about a half-mile or so in from the coast to the back of the valley. My wife who works from home will be surprised that it's foggy anywhere in Pacifica, as she's been enjoying hours of warm sunshine! So moral of the story: if fog matters to you in Pacifica, go south and east in the city as far as you can!