A lifetime cash flow forecast takes everything you have and everything you want, and runs it forward one year at a time. Income comes in, spending goes out, pensions and investments grow, the mortgage falls away, and tax is taken at every step.
The result is one chart. It shows the years your wealth grows, the years you draw on it, and whether it lasts as long as you do. If it runs short, you see the year it happens and the size of the gap.