 Therefore, it is paramount that one know the time period that the growing degree-day values are calculated from so as to be comparable.The Winkler index uses the standard method of calculating growing degree-days in viticulture and is based on using a base temperature of 50 °F (10 °C) with no upper temperature cut-off.The first issue is that 50 °F (10 °C) is not likely the best base temperature even though it is the most commonly used value.