The present note attempts to clarify the long standing topics of beam energy gain at FNPL. After a recalibration of the spectrometer magnet end of 2001, it was found that the beam energy was lower than what one would infer from the settings of the various radio-frequency components (the gun and the TESLA cavity). In this note we analyze these observations and propose a new rf-calibration (done with beam) for the rf-gun and the TESLA cavity.