What's a simple way to calculate magnification?
Take a picture of a ruler.
I would not try to calculate it. In the olden days you could do that, you could estimate it from the extension. But modern lenses are internal-focusing, and that changes the focal length. And we don't usually know what the current focal length is. So calculations are not helpful. Just photograph a ruler so it spans the long part of the frame. Then m = Sensor_Width_mm / Ruler_mm.