when what the user really wants to know is "How many more songs or photos can I store?"
That depends on how large they are, and that can vary widely, which is why abstracting away files (or more specifically, file size) is such a bad idea.
How many KM remaining depends on the terrain, traffic, driving technique, weather and a host of other factors. Nobody is expecting 100% accuracy just a rough estimate. They are already mentally doing the conversion anyways.
That depends on how large they are, and that can vary widely, which is why abstracting away files (or more specifically, file size) is such a bad idea.