A u-tube doesn't need calibration in the respect that the difference between the two scales will be twice the static suction. For convenience though, in order to read accurately from 0, it should be "calibrated", because some oil will slowly be lost over the years. Simply disconnect the tops of the tube and slide it until the oil level is even with the zero reading. Replace the connections.
How different the levels are will depend on the accuracy of the device and the size of the fan, among other factors. What's important is what was the reading when the system was new? That should have been documented after installation and posted. If the current reading is significantly different, the cause should be determined and corrected.