The offset issued was determined to be the input saturation of the PI box. Increasing the lock-in sensitivity from 500uV to 1mV solved the problem.
The new calibration constant is quoted from the second measurement which has a higher precision (relative error ~ 1 %).
The polarization after 2 EPR measurments is about 60%
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EPR Sweep #1
EPR AFP #1
EPR Calibration #1
EPR Sweep #2
EPR AFP #2
EPR Calibration #2