The two-part test
Was force necessary in the circumstances as you honestly believed them to be? And was the force used proportionate to the threat? Both questions must answer 'yes'.
Where guards get into trouble
Continuing to use force after the threat has ended. The law recognises the difficulty of judging in the moment but it does not tolerate 'punishment'.
Documenting force lawfully used
Immediate written note, medical check offered, CCTV preserved, statement to police if attended. The paper trail is what protects you weeks later.