How to calculate Actual sales YTD and Forecast for rest of rear in powerpivot

I have several files that I have compiled into power pivot and I am trying to create a formula to give me a Full Year Forecast field that I can use in my pivot table. The Full Year forecast would equal the sum of Actuals for the current calendar date minus 1 month plus the sum of Forecast for the current calendar month and each subsequent month through the remainder of the year.