Wow, I didn't know that! And yes, that's an impossibly subjective standard. It's such a minefield.

I'm not sure the standards of self defence for civilians would work, because the police so often find themselves in situations that no civilian would face. To be an armed enforcer of the law you DO need more latitude than an ordinary citizen.

But the standard is clearly slanted too far in the officer's favour. It's almost impossible for them to do something which is legally recognised as a mistake.