Great callout. DEM03 is a really important module. Let's take a look at the baseline forecast:
The correct formula is: IF 'SYS01 Time Settings By Week'.'1st Forecast Year?' THEN 'DAT03 Historic Volumes'.'Offset Volumes for 1st Forecast Year' ELSE OFFSET(Default Forecast, -52, 0)
Let's break that down:
First, we need to determine what time period we're currently on. Secondly, If it's the 1st forecast year we should grab the history and "seed" the future with historical data. If it's the second forecast year or beyond, we should just look back 52 periods. This essentially seeds the 2+ years with the same forecast.
The growth rate you've already determined in DEM02. The hint there is to use SYS08 to do the LOOKUP.