The book uses two key tools to get us to our Safe Spending Rate (SSR%), and they in turn get us to our Safe Spending Amount (SSA). The first is a Retirement Withdrawal Calculator (RWC). You’ll find several 100,000s in search results for that term on the internet, but we retirees must use calculators of a certain kind. The book relies on one workhorse calculator that tells us more than any other I could find and one other to verify its basic results. I’ll add a couple of posts on the details of how RWCs work.
The second key tool is a Probability of Living Calculator. As uncomfortable it is to think about our planning horizon, often called “Retirement Period,” we need to understand the probabilities that we (or at least one of us if planning for two) will be alive in the future. Our perspective of the number of years feeds into the decision of our Safe Spending Rate.