A very important aspect is to properly brief new trainees FO/Captains/Instructors on the fact that, unlike other "conventional" aircrafts where a little help is given to avoid digging a hole and spending the turn around with paperwork, on the Airbus FBW family there will be a takeover from the Captain/Training Captain should the need arise, that this is perfectly normal and there is no other way around. In my experience spending a few words with the trainees prior to their first flights and emphasising this aspect has proven beneficial in order to avoid potential demotivation and drop in performances following a low height takeover. It is then up to the trainer/captain to set their own threshold to what is acceptable and what is not according to each own's level of experience, but a takeover is always the right and only option if we are not happy with anything. It is better to have a takeover when it was not really needed than the opposite, and with time everybody can adjust their own tolerance.