The scaling ratio is use to adjust the growing degrees needed for a perennial crop to reach maximum leaf area index and consequently the maximum crop coefficient. The ratio allows adjustment of crop coefficients developed in hot environments to be used in cold environments. The ratio stretches or shrinks the GDD base line. To use the ratio determine the accumulative GDD for the growing season in the location where the crop coefficient was developed and the GDD in the location where you desire to use the crop coefficient. The ratio of these two numbers should be used to calculate the scaling ratio. To determine if the adjustment is correct the crop coefficient should read a maximum in July for a perennial crop.
To determine where the original crop coefficient was measured see. http://weather.nmsu.edu/Crop%20Info%20Menu/index1.htm
The crop coefficient functions are listed under the individual crops.