OK well as expected this falls into the category of YYGT (Yes You Get That) 
Seriously though a reading of 5 and a max of 3 and you get greater than 100%. This happens to most folks whether or not they have the solar sensor shaded or not. The design of a Davis solar sensor is one which measures global solar not direct, that is it reads the whole sky including any reflections etc. This makes life difficult when you are estimating the sunshine as a % from actual w/m