Two critical retirement planning questions you need to make assumptions about are - how long will I live and how well will I live? We don't have the answers, but there is now more information on the possibilities and probabilities. We need to plan and save for how much income we will need in our later years when we stop working, but we don't know, with certainty, when we will die. So this critical financial planning still involves quite a bit of guesswork, but expert insights can help you avoid costly mistakes.