What do you like to use, to fit things into the small dimension of the 135 format frame at the standard working distance which you enjoy? Simply use the same multiplier, except use 90mm rather than 24mm frame short dimension. For example, if you like using 75mm for environmental portraiture on 135, 75/24 = 3.125; 3.125 * 90mm = 281mm on 4x5 format.