When you install a piezometer you want to take a reading at the ground surface so you know what your atmosphere reading is, this will always be subtracted from your measured pressure to obtain the water pressure value. There is also a zero reading included on the calibration sheet taken at the manufacturer, but it will always be slightly different based on the elevation of your location.
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You also want to take another reading with the sensor submerged in water as your pre installation check, this is verify the sensor responds to the increased pressure from being submerged in water. You can also use this opportunity to saturate the filter stone at the tip of the sensor. To do this remove the filter with the sensor submerged, tilt the tip up to let any trapped air escape and then replace the filter stone under water. You can then leave the piezometer in the water until you are about to drop it in the hole.
Hopefully you did all this and then recorded the ground surface elevation and the depth to the tip of the sensor as it is installed. With all this information you can calculate the following using this generic equation:
pore pressure = (raw_reading - surface_reading) * calibation_factor
water level = (raw_reading -surface_reading) * calibration_factor * pressure/ftH2O + ground_el - piezo_depth
1 ft h2O = 9.8 KPa
1 ft h2O = 2.306 psi