I addressed an almost identical question over the weekend. Please take a look at the following thread, as well as the presentation I gave at last week’s Analytics Summit that provided the background for the thread below:
My strong recommendation would be to create your forecast table in Power Query, so that you can do the allocation there as well (since PQ can’t access DAX calculated tables). Because DAX calculated tables update only on refresh or initialization of the report, they are equally dynamic as PQ-created tables.
I hope this is helpful.
- Brian