Hi MaddieGreen
There is no standard functionality to update demand forecast with past sales in BC.
The process of updating the Demand Forecast line is manual, unless of developing an extension.
In my previous implementations for Food & Bev companies I have used two different approaches to this problem:
1. Copy the demand forecast from Sales Budget with type = Quantity.
If the organisation have an S&OP plan, they are likely to have sales forecasts.
If the sales forecasts (Sales Budgets in BC) is updated, the extension can copy the Sales Budget Lines to the Demand Forecast lines with a routine.
For companies in the FMCG industry this can be once a day.
2. Copy the demand forecast from the Item Register to base forecasts on past sales (Not my preferred option)
If you work mainly on past sales, the Item Register report holds data about past sales that can be copied to the Demand Forecast lines.
I don't usually recommend this option because you will not have visibility of lost opportunities due to shortages.
You Side Question: I would not use Fixed Reordering Qty if your demand changes often. Instead, try Maximum Qty or Lot-for-Lot.
Then you can setup different Time Buckets for each line depending on stock obsolescence, I.E. products with short shelf-life can have a 1D time bucket while products with longer shelf-life can have 1W.
Hope it helps.
