Yeah that's the thing, the foul line or some other reference point would be a decent starting point as long as every hitter stands in the exact same place in the batters box. I know they all tend to stand at the back of it these days, but for another batter who stands even a foot closer to the pitcher, his "half-swings" would be way out of whack by using the foul line.
That's the inherent problem with this kind of call when using some sort of physical marker on the field as a guideline. I'm sure umps want to get the calls correct, but it's been demonstrated time and again that in certain situations, umps have a tendency to err on the side of wrong. The strike zone mysteriously expands in a 3-0 count for example.