I too have the same CA813 meter, it's good to hear that yours is spot on, gives me hope mine is to.
Craig, which 'source' setting do you use your 840020 on? Choices are florescent, mercury, sodium and tungsten/unknown. If I use the latter I get a much lower reading than my CA813.
Did you get a 840022 ? http://www.sperdirect.com/scart/public/database/repository/other/Instruction_Manuals/840022-i.pdfYou might call Sper and ask them.