For a situation to be comparable, it would have to include a physical altercation. In this situation, the gun wasn't being pointed at the officer from 30 feet away. He was going for his gun (if that is what occurred) during the struggle. Guns are pretty accurate at that range, and the officer's life could have ended instantly. In that situation, the officer has defend himself and use as many bullets as necessary to end the threat. Firing a warning shot or a wounding shot is not an option.
Again, that's IF he was going for his gun. If he wasn't, I'm not sure how it can be justified.
To my knowledge there is no specific protocol about how far a person with a deadly weapon can be before the police engage.