Julian,
As already mentioned by Krishnendu there is no provision to change the forecast formulae for calculation in standard design of the system.
I do not think you can even modify the calculation with any enhancement but explore the options where you can modify the forecast results before you SAVE or Before your display.
Check below customer exits with your technical team.
MCP20025 User Exit for Saving Forecasts
MCP20022 User Exit for Displaying Forecast Results
BR
KK