alt Hacker Newshuh, yesterday I vibe coded a similar tool. It lets me set my current savings, investment, retirement. Expected yearly savings, expected number of years of work left. After retirement, only withdrawals are allowed (investments, savings still get returns tho)
Codex suggested a bunch of improvements: monte carlo, investment volatility, expense inflation, capital gains tax etc. One thing codex didn't ask about was social security. So that's another factor that could be improved.
I wanted to know 'do I really need to work 30 more years?'
What would be really interesting is discovering an attainable formula where my investments outpace my expenses. Or, what variables let me die with basically $0 in the bank
At the end, I realized I had no idea what the calculator was REALLY doing. So it could be right... it could be really wrong too. Having historical data might help solve those issues
Your last paragraph is why I did using MC simulations based on credible data. Once you pick a return and a volatility, the output just restates your inputs and you can't tell a wrong answer from a right one. Historical data doesn't fix that, but it moves the assumption somewhere you can argue about, and shocks arrive as correlated bundles that actually happened.
Dying with roughly $0 is a real mode in the app: floor and ceiling, or Yale style smoothing, where spending flexes instead of staying fixed.