My question is, when figuring out Guide numbers, using GN= shooting distance x aperture / ISO factor, what factor should I use for ISO 160? If there is no ISO factor for 100 and 1.4x for 200, should 160 fall at about 1.2x? Also, if the ISO factor for 400 is 2x, what is it if you rate 400 speed film at EI320? Or would it just stay the same? Thanks alot.