Occasionally when I run into a scene that has too wide a contrast range, I pre-expose the film on a zone three value through a diffusion panel that I carry with me. This is a way of breaking the film threshold and moving the low values up on to the straight line portion of the film characteristic curve. I meter this exposure by placing the diffusion panel over my meter lens and after arriving at a Zone III exposure I then place the diffusion panel over the camera lens for the first exposure. After the initial exposure, I then meter the scene as I normally would and make a second exposure on the same sheet of film without the diffusion panel.
This can best be illustrated by assigning a numerical value of 1 for Zone I, a value of 2 for Zone II, and a value of 4 for Zone III. As we double the values for each subsequent Zone we, in making this preexposure, add a greater proportional value to the low zones and a very small proportional value to the upper zones. This would effectively expose a Zone III luminescence to a Zone IV value but have virtually no effect at the Zone VIII value. (effectively adding value of 4 to a VIII value of 128).
I find that this is a way to effectively support the low values, to bring those values up on to the straightline portion with the better separation that this portion of the film curve supports, and effectively compress the scene contrast range without doing a minus development of the film.
This is lesson eight from my zone system workshop. Good light and good fortune.