Creating forecasts by applying a factor to existing forecast.

I want to create different forecasts based on applying a factor to a 'given' forecast.  

- The factor is based on the average of cumulative actuals/planned (milestones)

- Forecast 1 will go for 2 months (after actuals) and it is based on the 'given' forecast * the average of cumulative actuals/planned over last 3 months (factor)

- Forecast 2 will start after the Forecast 1 and is based on the 'given' forecast * the average of cumulative actuals/planed over last 12 months

I came up with the following measurement, but I have to calculate the 'factor' outside of Power BI.  Also, I am not able to specify the months to apply the factor to. How can I do these forecasts without using a plug 'factor'?

This is an example spreadsheet of the problem:

Thank you