a. Setup > Company > Enable Features > CRM, Advanced Forecasting=True under Sales.
b. Setup > Sales > Sales Preferences > Forecasts tab, Low Forecast Name=Worst Case, Medium Forecast Name=Most Likely & High Forecast Name=Upside
The computation is not based on how many estimates are issued against an opportunity but it is based on the Forecast Type used in each estimates.
This is calculated based on this rank when viewing an existing opportunity under General tab > Estimates subtab:
a. If there is no Forecast Type=Most Likely, then the next amount to be shown is the Worst Case.
b. If these is Forecast Type=Most Likely, then only the Most Likely amounts will be shown.
Note: Omitted and Upside are not included in the computation.