For retirement calculators monte carlo simulations seem like the best way to model risk, but all existing implementations are several orders of magnitude slower than they should be. I made https://james.darpinian.com/money/ to prove that it's possible to simulate a million scenarios in a few milliseconds. You can drag sliders and see simulation results update in real time. Even on phones.
Because I made it for my own use, the UI is terrible. The simulation result you need to pay attention to is the % success at the bottom, which changes as you drag the sliders. The simulation assumptions are reasonable and sophisticated (more so than many existing FIRE tools, especially the use of actuarial tables) but not documented except in the code comments. It would be cool if one of the real FIRE calculators out there with a real UI would implement a fast monte carlo simulator like this. I'd be happy to share how it works.
Because I made it for my own use, the UI is terrible. The simulation result you need to pay attention to is the % success at the bottom, which changes as you drag the sliders. The simulation assumptions are reasonable and sophisticated (more so than many existing FIRE tools, especially the use of actuarial tables) but not documented except in the code comments. It would be cool if one of the real FIRE calculators out there with a real UI would implement a fast monte carlo simulator like this. I'd be happy to share how it works.