Hello Michael
As I mentioned earlier, forecast in one plant does not automatically reduce the forecast on another plant.
You can however, create a scenario where forecast is created in one plant, then MRP generates stock transfers to cover this forecast and the stock transfers reduce the forecast on another plant. In this case, I suggest you to use strategy 40 instead of 63. You will find several threads and documents about the forecast consumption by stock transfers on SCN.
BR
Caetano