The computational methods for calculating the properties of glazing systems containing shading from the properties of their components have been developed, but the measurement standards and property databases necessary to apply them have not. It is shown that with a drastic simplifying assumption, these methods can be used to calculate system solar-optical properties and solar heat gain coefficients for arbitrary glazing systems, while requiring limited data about the shading. Detailed formulas are presented and performance multipliers are defined for the approximate treatment of simple glazings with shading. As higher accuracy is demanded, the formulas become very complicated.