The tool should have a separate forecast module where you enter the range of yearly % returns for each asset (MAX & MIN) and the tool runs a Monte Carlo simulation to find a probabilistic future value of your portfolio.
This would be used to calculate the expected portfolio value at a very future date for retirement planning / FIRE analysis.
The initial implementation would use a default trigen distribution based on the max/min, but future updates could include different distribution models to try and reflect what an individual investor believes to be a likely performance of a given investment.