How to define, measure Shading Coefficient

[Sparkco]

Hi friends, I would like to know how to define Shading Coefficient. I know the SC for thermos glass has already defined by the glss manufacturer, but what about if I have an overhang on top of the window, and the SC will be different. Is it the only way to define the SC only by physical measurement ( using photo sensor )??? In order for me to calculate the solar heat gain, I need to know the SC.
Thank you for your help.