The duration of a pre-function is "predicted" by the FGC doing a basic calculation dt= (L*di)/(V+RI) and hence the FGC knows when to start the pre-function in order to arrive on-time for the beam extraction.

However, in case when a heavy magnet saturation takes place (typically at high current only), the calculation is not correct and the current arrives at the target "user value" earlier. This can be diagnosed using the G REF.CYC.PRE_FUNC_ERR which returns a large negative value (time in seconds that indicates how early one arrived at the function value).

There is nothing to be done for this problem and it practically means we will always be early for the function. If the converter is stable the inductance value can be reduced so the above calculation yields a shorter prefunction estimated time.