Whilst not an exact science this is a tool that I use at work to help people visualise their financial future. Most ventures use projections based on assumptions when planning the future. It informs decisions and helps work out whether a risk is realistic or reckless.
Professional Paraplanner have posted a few insights from me which might be interesting. Whilst cashflow is based on long term (generally linear) assumptions likely to change in the future it is nevertheless a useful tool and a great conversation starter.