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.