How does that even work - do you actually draw a mental image of yourself walking up that person's length foot by foot? And what if your feet are not 30cm, but

How does that even work - do you actually draw a mental image of yourself walking up that person's length foot by foot? And what if your feet are not 30cm, but longer or shorter? If you say "about x" you might as well estimate to 10cm precision and say "about 180cm" or "about 190cm".