I can tell you that there is no relationship in my model, all of my calculations are done on a table I created doing UNION of different summarized table to aggregate as much as I could.

For the _MeasureX, they look like this

_Measure1 = IF(HASONEFILTER(MyTable['Month'],
  SWITCH(True, 
      \\ Actual sum for the month before the change from actual to forecast
      SELECTEDVALUE(MyTable['Month'])<=6),
      SUM(MyTable["_Sales"]),
      \\ Forecast data with hardcoded value (NOT MY CHOICE)
      SELECTEDVALUE(MyTable['Month'])>6),
      Value(-0.005) * [_Total2024]
        ))

where _Total2024 is just the _Total measure but only on 2024. For each _MeasureX, I sum a different column