I have a deep set of spreadsheet tabs that track retirement funds in many different ways: bond ladders, stock investments by sector, market cap, investment thesis, stocks vs bonds vs alternative assets, security type, etc.
However, the one thing I couldn’t model easily was future returns that take into account the variability of the market. While the long term returns of stocks might be 7-8%, for example, you can’t just plug that into the spreadsheet model. You have to model variability because it will impact greatly the odds of running out of money or not. The impact of variability is significant and can’t be ignored in many retirement scenarios.
So for that reason I use financial calculators to run Monte Carlo analyses. However, I make sure to throughly understand the calculator assumptions. Many, for example, currently project far too optimistic projected returns from stocks. Given market valuations, a 10-11% return from stocks would seem to be far too optimistic. I like calculators that let you adjust the projected returns and then run Monte Carlo analysis on that projected return. Also, I model sequence of returns risk to see the results too.
Unlike most, I never sat down and tried to fully understand all of the math of retirement planning while I was working. I simply saved as much as possible and then accidentally fell into retirement due to a health issue. At that point I then dove in deep into investment management and planning. Fortunately we had saved enough.