It may be helpful to understand why you add 5 to the difference in stops. 5 has been chosen because the greatest difference between the whitest white and darkest black that can be created with paint is approx. 4 1/2 stops rounded off to 5 for nominal use. Therefore if the difference between lit and shadowed areas is two stops you have the potential for 7 stops of difference between the whitest white in light to the darkest black in shadow. Such differences may not exist in the actual scene being photographed but are provided for in the exposure and development chosen.