You should be checking your 301 regularly with the 301-27 calibration strip. It is an 1008 NIST traceable strip with known values. It should be replaced annually.
The calibration strip is used to veryify the unit's accuracy and to adjust for changes in lamp strengh.
If your daily numbers are off the first thing to do is check the instrument with the calibration strip.
With a 2mm aperture you are allowed +/-.02 to the calibration strip and can measure up to 5.0D