ADXL345_pitch_roll.ino calibration

Hi,
i'm trying to run ADXL345_pitch_roll.ino and it works, but i never obtain 90degrees in X axis and i obtain 105degrees in Y axis when it's really 90degrees.
I think i have to calibrate, can you explain how to do it? i read about Aoff and Gain here
https://www.analog.com/media/en/technical-documentation/application-notes/AN-1057.pdf

what do you think about?
Thank you so much