That's exactly what I've been taught, too. You can only use defense which is proportionate to the offense.   A number of years ago a chap in Indianapolis had a flat tire in a bad part of town. Some African-Americans were standing about 75 feet away -- he "felt threatened" and shot and killed one. The Indiana Supreme Court might have its faults, but they upheld his sentence.

You can also take into account "discrepancy of force" -- someone in a wheelchair would have greater latitude in self-defense against a healthy street thug than I, as an able-bodied person, would have.

Things can be replaced, lives cannot. Taking a life should only be done under the greatest duress.