I think a reduction key is used when netting real orders from forecast.
Instead, I would look at a Period key. They are configured at Master planning > Setup > Demand forecasting > Period allocation categories
If you create a Period key with the Lines shown in the attached picture and add it to your Demand forecast lines, it will spread your monthly forecast over four weeks. This way, the entire months forecast will not disappear on the 2nd day of the month.
If you're uploading your forecast lines, its easy to set a Period key during import.
If you want to test it by manually setting a Period key on your Demand forecast lines, it's harder.
On the Demand forecast lines screen, on the General tab, you'll find the Period key field, but for me it is read-only.
Add fields called Method and Period key on to the screen using Personalisation. For some reason, these will be editable.
Set the Method to Key and then select your Period key.
Press Save.
The Allocation grid at the bottom of the screen will update automatically.
