[sudosysgen]

Revolve the entire setup around the axis perpendicular to the slit and you will have a very accurate representation of how a camera-lens system works.

The ratio between distance and focal length only works if the focal lengths are equivalent across the two cameras. Otherwise it doesn't work. That's to say, a 70mm f/2.8 has the same depth of field as a 35mm 1.4 lens if the second is on a camera with 2x crop factor.

Try it out, crop the image of your 70mm lens at f/2.8 and compare it to the image of your 35mm f/1.4 lens and you will get exactly the same image with the same blur (assuming the lenses are exactly 70mm and 35mm at the focus setting, which is not guaranteed due to focus breathing and manufacturers rounding off their focal lengths)