The actual time step length can only be determined in the OnTimeStepConstraint callback, which might be called several times in a time step. FEFLOW itself cannot know what the final length of a time step will be before it is calculated as - using the predictor-corrector scheme - the simulation may be carried out several times with different time step sizes before actually accepting the result. PreTimeStep will only be called once, so it will not be useful for you. OnTimeStepConstraint, however, is called before the time step is actually simulated, so this should be where you actually put your code. Keep in mind, though, that it may be called several times before FEFLOW proceeds to the next step.
PS: Merged your two topics.