TMC4671-How to calibrate BISS C encoder electrical angle offset when initialize BISS encoder?

Now in our project, we use ST MCU to configure TMC4671 to control Stepper Motor with a Biss c encoder.  I do not know how to get the phi_e  offset  when i initialize encoder ?

I also have read the datasheet of TMC4671 as below, i  am confused with Bang-Bang Initialization, how to opertate in code ?

3.5.7.2 Bang-Bang Initialization of the Encoder

A Bang-Bang initialization is an initialization where the motor is forced with high current into a specifific

position. For Bang-Bang initialization, the user sets a current into direction D that is strong enough to

move the rotor into the desired direction. Other initialization methods ramp up the current smoothly

and adjust the current vector to rotor movement detected by the encoder.