Calculating forecasting of Inventory?

How are you Calculating forecasting of Inventory? I have heard of different ways, but curious what you have found to be successful. Looking for advice.

We use the current inventory, plus known purchase orders from a stream, less expected Sales from a Budgeted Sales stream.

Obviously at some point you run out of one or the other.

Would you be willing to share a screen or two of how you have this setup. I would like to try and duplicate to see if it will work for our needs. Our inventory data is only a rolling 12 months which I assume is fine for this.

Not familiar with Budgeted Sales stream. Thanks for any additional information you can provide.

Were you able to get this set up?
Im also looking to generate a forecast of inventory but using different criteria for the outbound inventory.
does the calculation have to occur in Phocas or should it occur in the SQL?