[Brightcolours]

   Hm....  For macro lenses at MFD (_well, usually_) we have ratio 1:1, so 36mm object projects on 36mm sensor at the MFD, and what actually means the same FOV for different macro lenses at this magnification (1:1).

You are confusing main subject size and FOV. FOV is about the angle of the view, not the size of the subject (with varying distance..). So no, not all lenses at 1:1 have the same FOV (not even close, actually). 

Sensor size doesn't change, and we know both MFD and magnification (and thus max. object size, that could be projected on the sensor at that particular distance) for every lens from manufacturer's specifications. So, we have isosceles trapezoid  with further calculations arising from it, instead of classic calculation of the FOV, where we need FL. Of course, indicated magnification may not be very precise (Popphoto tests), but we can measure it more easily than real FL, and on this approach we even do not need focal length of the lens.

You can indeed calculate FOV with subject size and distance to the subject (although even this is tricky, as for distance to the subject you have to use the focal point to get it exact. And that focal point is somewhere within the lens system, and hard to determine. Some use the aperture position as seen from the front of the lens).

    Unfortunately, I am not so good in math to further elaborate that.

 

A.