Another tip, in addition to Chan's - let the light source stabilise by leaving it on for a few minutes before calibrating, and again for a few minutes before taking readings.
If a step wedeg is not handy, get a start with theatrical gel - rosco makes a free sample book of a whole slew of colours. Near the back there are 1/3, 2/3 and whole stop neutral denisty gel samples. . Each whole stop is 0.3 I recall. Pull then from the sample book, and stack them to create the appropriate density to get north of 2.0, to set the slope. You may want to start with a piece of film base and zero before adding the ND tothe film base and set the slope. then tape over the slope knob. It should not change .