man, nobody told you but its not a constant precise crop formula, its total manual adjustment of what acmera lets us record and card speed factor.
Sometimes you chop off pixels from crop modes to get more recording time.
its a hack, if we will somehow get even more speed from cards, the crop will get more pixels and field of view will be bigger, with slow sd cards crop gets smaller part of full resolution image.
easiest thing you can do, compare full resolution photo size to raw mlv resolutions.
but remember that crop modes are not centered super precisely into the middle of the sensor
your math does not apply, you can divide full res photo by 3 , and even then some pixels can get discarded depending on what ml coders choose.