Here's my table to determine bellows factor by magnification. It's small, so I have them taped to the backs of all my LF cameras, on my light meter, and in my notebook. To determine magnification just estimate the width of the scene at the subject position and compare to the width of the format, and then use the table to find the bellows exposure factor--works for any format with any camera. For instance, if you're taking a headshot with an 8x10" camera, the distance from one shoulder to the other will be around 24", and the format in portrait mode is 8" wide, so the magnification factor will be 1:3, and you'll need about 2/3 to 1 stop additional exposure. For the same image on 4x5" the magnification factor will be 1:6, so you would need about 1/3 to 1/2 stop additional exposure. If you want to be more precise for higher magnification factors, you can put a ruler in the scene.