Hello
I would like to let you know - just in case you will come with the same issue/task - the only way I could get the calculation in DAX for AVERAGEX for a mix of actual and forecast data was through Power Query. I marked rows for each forecast version, ie. 2+10 would have all actual from Ledger A for period 1 and 2 and 10 months from Ledger F for period 3 to 12, etc. Via list functions. Then from the row context it was easy to calculate AVERAGE of mixed data.
Kind regards
Mira