The 1.5 you are thinking about is the log-H range used to determine the average gradient when doing fractional gradient testing. How it relates to how the manufacturer gets the ISO rating is what I'm talking about. By knowing how the details, parameters, and theory on a test, the photographer can better utilize the testing data. There will be a slight difference in the film's responce when testing under an overcast day and sunny day. the ISO speed standard uses daylight color temperature for exposure. When the speed standard changed the color temperature for the film exposure from sunlight to daylight they had to adjust the speed constant from 1.0 to 0.80. But if the photographer is testing using natural light, they are probably using a camera. All the variables associated with camera exposures will tend to obscure and speed change caused by different color temperatures.
It also relates to how the manufacturer gets this ISO rating and what it means. As I understand it and I may be wrong here but the rating correlates to a contrast of 1,5 am i wrong here? or me that is an overcast day in winter. Not a sunny day when many people take their cameras and run on the streets.