Another thing you will have to take into account is that when using the monorail to take full frame Head & Shoulder portraits you are likely to be closer than the 8-10x focal length distance (at which it is deemed not to require exposure compensation) which will require you to make an exposure compensation for the length of the bellows (increased fall off of light).

The easy way to do this is to take the focal length in inches (say 10" 250mm) and the length of the bellows between the standards in inches (say 14"), transpose the two figures into f nos f10-f14 and calculate the difference between them, in this example it would equal approximately 1 stop.

200mm lens (8") with 13" bellows would give f8-f13 = approx 1 and 1/3 stops to be added.