I received advice from Western digital once that one should NOT salt test digital hygrometers as the salty moisture could promote corrosion. My understanding is the best calibration is done lower than the RH provided by a salt test (which typically is around 75% RH). I would use a boveda packet (either 60, 65 or 70, depending on what your target RH is for the smokes).
Also remember RH is temperature dependent. Any given RH level (as shown on the hygrometer) will affect the actual amount of moisture in your cigars depending on whether the room is at 50, 60 or 70 (*F) degrees. The RH of the room your humidor is in will also affect the amount of moisture in your humidor. Humidors are almost never moisture- and air-tight and moisture content of air seeks balance, similar to how heat and pressure regulate.