Ratio is a scaling factor to adjust the development rate and crop coefficient of a plant based on Growing Degree Days. This adjustment is needed for an perennial crop that has a total growing season that varies from location to location. An examples is grass at Farmington NM  that grows from May to Sept  compared to that same grass that grows in Las Cruces NM from March to November. The ratio is used to correct  perennial crop coefficients that were developed at location other than where they are going to be used to compute Et. Consequently, for Mesquite the crop coefficient was developed in Arizona that has twice the GDD for the growing season compared to Las Cruces NM. To use the Mesquite crop coefficient in Las Cruces you must put in a ratio value of 2. Adjust the ratio until the crop coefficient peaks in July. Make sure that the crop coefficient does not decrease after July and then rise again in Sept or Oct. Because the polynomial is a third order it can increase in November is the ratio is wrong.