Modern densitometers often have a "null" mode relative to the light source, so if a little dust gets on
the receptor, it is ignored. Actual calibration requires an expensive reference standard - a very expensive kind of step wedge - and not just an ordinary one on film. But you can use your ordinary step wedge to check for any drift. I once worked quite a bit with spectrophotometers, and these had to be routinely recalibrated to a fade-proof standard, typically a white ceramic tile. I've been working with
the same pair of densitometers for several decades, and they are still within .01 measured density -
below the advertised plus/minus accuracy factor!