A different, but closely related question: Rather than consider lives in isolation, for what x do we prefer
a world which has a 1-x chance of drastically reduced starvation and disease and other effects of charities with easy-to-measure outcomes, and an x total chance of being destroyed by all x-risk factors
over a world in which there is a 1-epsilon chance of modest drop from baseline starvation and disease, and epsilon chance of being destroyed by an x-risk factor?

It is rational to have a preference for taking the riskier choice even for a large x, if one values quality of life over certainty of life.