Ideally, the level will be set to what will produce the best outcome based on our current understanding of the world, where the precise parameters under which 'best' is determined is negotiated through a fair political and/or economic process.

In practice, the question is how we best approximate that. The shape of that should be informed by economics, and will likely depend on the particular country.

All of that said, I agree it's a key factor to look at when assessing a particular BI proposal; I don't think it's an unanswerable question.