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.
The value of 0.62 for contrast index that you said was standard in most books, I believe came from the contrast parameters defined in the ISO speed standard. The delta 1.30 log-H by delta 0.80 density has a gradient between 0.61 and 0.62; however, it can't be referred to as a contrast index because the use of that term implies a testing methodology. The 1.30 log-H range is intended to define the shadow portion of the curve and not the average gradient.
Now, a CI 0.62 is perfectly acceptable as a normal. It's just a little high to be considered the results from the statistical average conditions. Most books have aims more around 0.56 and 0.58 depending on the average flare factor - whether it's for a large format lens or medium and 35 mm lens.
My basic premise is that there is good testing and bad testing. The best possible decissions come from a position of knowledge.