Subject: measuring exact focal length of camera lens

How do I measure the exact focal lenght of a "24 mm" camera lens?  Camera 
lenses have some deviation from their labled focal length.  The exact 
measurement is needed to construct a panoramic camera.  The camera lenses are 
designed so that the distance from lens to film is a constant 42 mm for all 
lenses for this camera (Canon) regardless of the focal lenth of the lens or 
distance to subject.  I don't have any optical equipment.  Is there a formula 
where given (for example) a 1 meter ruler photographed at 3 meters distance 
will produce a ??mm image on the film, and the exact focal length of the lens 
could then be calculated?

Re: measuring exact focal length of camera lens