You can calibrate your densitometer with any transmission step tablet (like the Stouffer), but only if that step tablet has itself been calibrated, because you must enter those values into the densitometer during the calibration process. If your step table has been calibrated, it may only have been calibrated in visual units, and not UV units, and if so, you could only calibrate the densitometers visual channel.
X-Rite sells a step tablet calibrated for both visual and UV, and they may have the manual for your densitometer online, to help with calibration.