[SilasX]

That doesn't work for the reason above: I don't want my grandma imprisoned, but zero imprisonment is also impractical. [1]

"A society is judged by how it treats its weakest", "would I want $GOOD_PERSON to fall victim to this" -- those sound nice, but don't translate into a practical metric for appropriately navigating real moral dilemmas.

[1] I know the lectures about the evils of mass incarceration; those don't prove that no one should be imprisoned.

[gervase]

Perhaps an extension, then: "Would I want this to happen to $LOVED_ONE, if they were convicted of $HEINOUS_CRIME?"

This may permit consideration of the relative social value of the punishment, alloyed with the emotional bias provided by the context.