Usually when calibrating any device, you would take a pure sample (000-001) and the higher number of the spectrum (say 10,000). A few other samples of say 010 and 050 just to make sure that you are not too far off. I think the price of a fixed meter is around the 30 quid mark, so I wouldn't go to all that trouble just to recalibrate your meter. They are calibrated at the factory, so to open up one of these devices would probably make less sence once you have them all back together again/time etc.
It would probably be quicker to wash your windows to see the outcome!