Move a pen on the card until the point of the pen disappears in your blind spot. Mark the places where the penpoint disappears. Use the pen to trace the shape and size of your blind spot on the card. Measure the diameter of the blind spot on the card. In our simple model, we are assuming that the eye behaves like a pinhole camera, with the pupil as the pinhole.
Light travels in a straight line through the pupil to the retina.
Similar triangles can then be used to calculate the size of the blind spot on your retina.