The point of those kind of estimation methods is not so you’ll agree with a clock, it’s so you’ll agree with yourself if you do it twice. Using a “foot” as a literal measuring unit isn’t so dumb if only one person is measuring.

In cooking, what a “cup” is in volume can vary by like 30% from cook to cook and depending on environmental factors. But if you’re the same cook, working in the same environment, your method will work every time.