How to calculate the sunshine percentage?

Sunshine percentage, that is, the ratio of actual sunshine time to possible sunshine time (sunshine hours that should be available when there is no cloud all day). It shows how much sunshine time (%) is reduced by climatic conditions (mainly clouds, rain, fog, dust, sand dust, etc.). For example, the actual sunshine hours in Guangzhou, Shanghai, Beijing and Harbin account for 43%, 55%, 63% and 60% of the possible sunshine hours in this area, that is, the weather conditions have reduced the sunshine hours in these four cities by 57%, 45%, 37% and 40% respectively.